6 Ways to Secure Your Business VoIP System Against Cyber Attacks in 2026

Business telephone systems have changed dramatically.

Traditional phone lines have increasingly been replaced by VoIP — Voice over Internet Protocol — hosted PBX platforms and services such as Microsoft Teams Phone.

The benefits are obvious: employees can make and receive business calls from desk phones, laptops and mobiles, numbers can follow users between locations, and businesses can add extensions or sites without installing traditional telephone infrastructure.

But VoIP is also part of your IT environment.

That means it needs to be secured like one.

The NCSC warns that compromised PBX systems can be abused for fraud, denial-of-service attacks and other malicious activity. Its current guidance recommends controls including secure configuration, restricted network access, monitoring, patching, encryption and disaster-recovery planning.

Here are six practical ways to improve VoIP security in 2026.

What Can Attackers Do With a Poorly Secured VoIP System?

VoIP attacks are not limited to somebody listening to telephone calls.

Depending on the system and how it is configured, attackers may attempt to:

Make unauthorised calls.

Generate expensive international or premium-rate charges.

Compromise administrator accounts.

Redirect calls.

Abuse call forwarding.

Intercept unencrypted communications.

Disrupt telephone availability.

Exploit vulnerable PBX or handset firmware.

Use the telephone platform as another route into the network.

The NCSC specifically highlights PBX fraud, where attackers exploit telephone systems to place calls that ultimately become the bill payer's responsibility. It also recommends monitoring for unusual patterns such as long-duration calls, unexpected high-cost destinations and changes in after-hours calling behaviour.

So VoIP security is not merely about protecting conversations.

It is about protecting availability, identity, network access and money.

1. Secure the PBX, SBC and Management Interfaces

One of the biggest risks is the administration layer.

Depending on the system, an administrator may be able to:

Create extensions.

Reset credentials.

Configure SIP trunks.

Change call routing.

Enable forwarding.

Modify international calling.

Access voicemail.

Alter firewall or network settings.

That level of privilege deserves proper protection.

Remove Default Credentials

Many network and telephony devices are supplied with default usernames or passwords.

Change them during installation.

The NCSC specifically recommends changing default passwords across on-premises PBX components and VoIP devices.

Do not leave:

admin / admin

or a password printed in a manual controlling business telephony.

Use Strong Authentication

Where your VoIP provider or management platform supports MFA, enable it for administrator accounts.

Also use separate administrator credentials rather than sharing one account among several engineers.

That gives you better accountability and reduces the impact if one person's credentials are compromised.

And one important correction to older VoIP advice:

Do not force employees or administrators to change passwords every 90 days simply because the calendar says so.

The NCSC recommends against routine password expiry because it can encourage weaker, predictable passwords and provides little security benefit unless compromise is suspected.

Instead:

Use unique passwords.

Store them securely in an appropriate password manager.

Change default credentials.

Change passwords when compromise is suspected.

Use MFA or stronger authentication where available.

Restrict Management Access

Ask whether the PBX management interface genuinely needs to be accessible from the entire internet.

If not, restrict it.

Administration may be limited to:

Trusted IP addresses.

A management network.

VPN or zero-trust access.

Specific administrator devices.

The fewer places from which a sensitive administration portal can be reached, the smaller the attack surface.

2. Segment VoIP From the Rest of Your Network

Your desk phones do not necessarily need unrestricted access to everything else in the business.

One of the NCSC's specific recommendations for an on-premises PBX is to isolate it using a separate VLAN or subnet.

That is good advice for wider VoIP infrastructure too.

You might separate:

Voice devices

Ordinary employee computers

Servers

Guest Wi-Fi

Management systems

Backup infrastructure

This can improve both security and network management.

If an employee laptop becomes compromised, network segmentation can make it more difficult for the attacker to reach telephony administration directly.

Likewise, a vulnerable IP phone should not automatically become an easy route towards sensitive servers.

Lock Down Firewall Rules

Do not simply expose every SIP port because “VoIP needs the internet”.

The NCSC recommends restricting inbound and outbound PBX traffic to trusted IP addresses such as known SIP providers, clients and other authorised systems wherever possible.

Firewall rules should be based on the requirements of the specific VoIP platform.

Avoid blindly copying port-forwarding rules from a ten-year-old forum post.

Session Border Controllers Matter

For some modern voice architectures, particularly Microsoft Teams Direct Routing, a Session Border Controller or SBC sits between Teams and the telephone network.

Microsoft requires a supported SBC for Direct Routing and uses SIP signalling over TLS between the SBC and Microsoft infrastructure.

That SBC should be treated as important security infrastructure — not simply a mysterious box somebody installed years ago and forgot about.

3. Encrypt VoIP Signalling and Voice Traffic

Traditional telephone conversations were largely separate from your computer network.

VoIP calls travel as network traffic.

That makes encryption important.

For hosted or cloud PBX services, the NCSC recommends checking whether providers protect signalling and voice communications with technologies such as:

TLS for signalling

and

SRTP for media streams.

TLS helps protect the signalling used to establish and control calls.

SRTP protects the actual real-time audio media.

For example, Microsoft Teams uses TLS to protect signalling and Secure Real-Time Transport Protocol (SRTP) for media. Microsoft states that SRTP provides confidentiality, authentication and protection against replay attacks for Teams media traffic.

With Teams Phone Direct Routing specifically, Microsoft documents SIP signalling over TLS and media using SRTP/SRTCP between its services and compatible SBC infrastructure.

Ask Your Provider

If you use hosted VoIP, ask:

Is signalling encrypted?

Is media encrypted?

How is the administrator portal protected?

How are certificates managed?

How are handsets provisioned?

Are remote users protected in the same way as office users?

Not every telephone platform works identically.

The goal is to understand exactly how your calls and administration are protected.

4. Patch the PBX, Phones, SBCs and Supporting Infrastructure

An IP phone is a computer with a handset attached.

It runs firmware.

The PBX runs software.

Your SBC runs software.

Network equipment runs firmware.

All of it can contain vulnerabilities.

The NCSC recommends keeping PBX systems and VoIP components up to date and enabling automatic updates where appropriate.

Your patching process should therefore include:

PBX software

Desk-phone firmware

Session Border Controllers

Underlying operating systems

Firewalls

Routers and switches

VoIP applications

Management components

Do not forget old phones.

An organisation might replace laptops every four years while an IP handset remains sitting on someone's desk for a decade.

If that device is no longer receiving security updates, it should be reviewed.

The NCSC has repeatedly warned that internet-facing and perimeter equipment can be attractive targets because these devices may run for long periods without the visibility and security tooling found on normal endpoints.

Maintain an Inventory

You should know:

Which PBX you use.

Which version it runs.

Which phones you have.

Which firmware versions they run.

Which SIP provider you use.

Which SBCs exist.

Who has administrator access.

When hardware reaches end of support.

You cannot patch equipment you have forgotten exists.

5. Prevent Toll Fraud and Monitor Calling Patterns

Toll fraud can become expensive very quickly.

An attacker who compromises a telephone account may attempt to make large volumes of calls or route calls towards expensive destinations.

A modern defence should combine restrictions and monitoring.

Restrict Destinations You Do Not Need

If your business never calls particular overseas regions, ask whether those destinations need to be enabled.

Different employees may also need different permissions.

For example:

A receptionist may need UK calls only.

A sales team may need international access.

A warehouse handset may need nothing beyond UK landlines and mobiles.

Apply the principle of least privilege to calling permissions just as you would to computer access.

The NCSC recommends restricting high-risk destinations and limiting call-forwarding or transfer functionality where it could be abused to route calls towards premium-rate or international numbers.

This is better than the old advice of simply blocking “1-900 numbers”, which is US-focused and not appropriate guidance for a UK business article.

Look for Unusual Behaviour

Your provider or monitoring system should ideally identify abnormal patterns.

Examples include:

A sudden spike in international calls.

Calls late at night when the office is normally closed.

Hundreds of short-duration calls.

Long unexplained calls.

Unexpected forwarding.

New destinations nobody normally contacts.

The NCSC specifically recommends monitoring call volumes, call patterns and system performance and securely retaining logs for investigation.

If your normal monthly telephone bill is £400 and suddenly the system generates £8,000 of overseas traffic overnight, you do not want to discover that three weeks later when the invoice arrives.

6. Plan for Failure — Not Just Attack Prevention

VoIP security also includes availability.

If your internet connection fails, what happens to your phones?

If your PBX provider suffers an outage?

If the office loses power?

If your firewall fails?

If the PBX is compromised?

If your SIP provider becomes unavailable?

The NCSC's denial-of-service guidance recommends that organisations understand their critical services and defences, create a response plan and test that response.

That principle applies particularly well to internet-based telephony.

Consider Internet Resilience

Depending on how important phones are to the business, options might include:

A secondary internet connection.

Automatic WAN failover.

4G/5G backup connectivity.

Call forwarding during outages.

Mobile applications.

Alternative numbers.

A documented emergency procedure.

One advantage of hosted VoIP is that calls may sometimes be redirected even when an office itself is unavailable.

But only if somebody has planned and tested it beforehand.

Back Up PBX Configuration

For on-premises systems, maintain backups of important PBX configuration and data.

The NCSC explicitly recommends regular PBX backups along with disaster-recovery planning and testing.

Know how you would rebuild the service if the telephone system failed completely.

Remote Workers Don't Automatically Need a Traditional VPN

Older VoIP advice often says:

“Put every remote VoIP user on a VPN.”

Sometimes that is appropriate.

But modern cloud voice platforms may already be designed to securely connect clients over the internet.

For example, Microsoft Teams protects signalling and media using TLS and SRTP, while Teams authentication can use Microsoft Entra ID, modern authentication, MFA and Conditional Access.

Adding a traditional VPN where it is not required can also alter network routing and potentially affect real-time media performance.

So the right rule is:

Follow the architecture recommended for your specific voice platform rather than automatically tunnelling everything through a VPN.

What About Microsoft Teams Phone?

For businesses already heavily invested in Microsoft 365, Teams Phone has become another major VoIP option.

It can provide calling through Microsoft's Calling Plans, Operator Connect or Direct Routing depending on requirements.

With Direct Routing, businesses can connect a supported SBC and existing carrier or telephony infrastructure to Teams Phone.

Security can also tie into the broader Microsoft ecosystem through:

Microsoft Entra ID

MFA

Conditional Access

Managed devices

Modern authentication

Microsoft's current Teams security architecture uses Entra ID for authentication and supports Conditional Access through modern authentication.

That can be a significant advantage compared with ageing PBX platforms where telephone security exists almost completely separately from business identity management.

Don't Forget Physical Handsets

Desk phones deserve attention too.

Review:

Default passwords.

Web-management interfaces.

Firmware.

Unused services.

Local administrator access.

Network placement.

Device replacement cycles.

The NCSC includes VoIP phones in its definition of enterprise-connected devices, alongside systems such as laptops, printers and video-conferencing equipment.

Treat them accordingly.

An IP phone is not merely a telephone.

It is a network-connected endpoint.

Should You Disable the Phone's Web Interface?

Not automatically.

The old version of this article recommended disabling VoIP web interfaces almost entirely.

That is too simplistic.

A web interface may be essential for legitimate administration.

The better approach is:

Disable it if it genuinely is not required.

Otherwise:

Restrict access.

Use HTTPS.

Change default credentials.

Patch the device.

Limit administration to trusted networks or devices.

The security principle should be reduce unnecessary attack surface, not blindly disable functionality without understanding what it does.

VoIP Security in 2026: The Six Priorities

A strong VoIP-security strategy should therefore concentrate on:

1. Protect administration

Remove default credentials, use strong authentication and restrict management access.

2. Segment the network

Keep voice and administration infrastructure appropriately separated from ordinary devices.

3. Encrypt communications

Use secure signalling and media technologies such as TLS and SRTP where supported.

4. Patch everything

Keep PBXs, SBCs, handsets, networking equipment and supporting software current.

5. Control fraud

Restrict unnecessary destinations and monitor unusual calling activity.

6. Build resilience

Have a plan for internet outages, system failure, cyber attacks and provider disruption.

No VoIP system is genuinely “bulletproof”.

A better objective is creating a telephone environment that is harder to compromise, easier to monitor and able to keep operating or recover quickly when something does go wrong.

Secure Business VoIP With Hamilton Group

Hamilton Group can help businesses deploy, manage and secure modern business telephony alongside the rest of their IT infrastructure.

We can assist with hosted VoIP, business phone systems, Microsoft Teams Phone, networking, internet connectivity, firewall configuration, cyber security, Microsoft 365 and wider managed IT support.

We can also review an existing VoIP environment to identify areas such as outdated hardware, poor network segmentation, weak administrative access, unreliable connectivity or inadequate disaster-recovery planning.

Because your telephone system should not sit outside your cyber-security strategy.

It is part of it.

And when your team needs IT support, our aim is to make first contact on support requests within 15 minutes.

Call Hamilton Group on 0330 043 0069 or visit hgmssp.com to speak with one of our IT and communications experts.