VoIP Security: How to Protect Your Business Calls from Attack
VoIP phone systems face specific security threats — including call interception, toll fraud, and phishing through voice calls (vishing). This guide explains the most significant VoIP security risks facing UK businesses and the practical controls that reduce them.
VoIP Security: Why It Matters
Toll fraud, call interception, and vishing are the primary VoIP security threats for UK businesses. Toll fraud alone can generate losses of thousands of pounds within hours of a system being compromised. AMVIA deploys SRTP/TLS encryption, SIP hardening, and real-time fraud detection as standard on all managed VoIP deployments.
Explore VoIP security optionsVoIP Security Threats UK Businesses Face
VoIP systems are a specific target for cybercriminals because they present a financially lucrative attack surface. Unlike stealing data, which requires monetisation, exploiting a VoIP system to make international calls generates direct, immediate financial losses for the victim — and the attacker has usually moved on before the fraud is discovered.
The most common VoIP-specific attacks affecting UK businesses are toll fraud, SIP scanning, vishing, and call interception. Each has a distinct mechanism and requires specific controls to prevent.
Toll Fraud: The Highest-Cost VoIP Threat
Toll fraud occurs when an attacker gains access to a business's VoIP system — typically by obtaining SIP account credentials through brute-force attack or credential stuffing — and uses it to make large volumes of international calls. International calls to premium-rate destinations, satellite phones, or certain country codes can cost several pounds per minute. An undetected attack running for 48 hours over a weekend can generate bills of tens of thousands of pounds.
Prevention requires strong, unique passwords on all SIP accounts, failed authentication alerts, blocking of international calls to high-risk destinations, and spending limits with automatic block triggers. AMVIA configures these controls as standard on all managed VoIP deployments.
Call Interception and Eavesdropping
VoIP calls transmitted without encryption can be captured by anyone with access to the same network segment — particularly relevant for businesses using shared or public Wi-Fi, or where the internal network has been compromised. Call content can include sensitive commercial discussions, personal data, and authentication information shared verbally.
SRTP (Secure Real-time Transport Protocol) encrypts the voice content of calls in transit, and TLS (Transport Layer Security) encrypts the SIP signalling that establishes and manages calls. Both should be enabled on all business VoIP systems. Hosted UCaaS platforms from reputable providers typically include both by default.
Vishing: Voice Phishing Targeting Your Staff
Vishing (voice phishing) uses phone calls to manipulate staff into disclosing credentials, authorising payments, or providing access. Attackers impersonate HMRC, banks, IT support providers, or senior management. This attack type has grown significantly — particularly targeting finance teams and receptionists. The NCSC and Ofcom have both issued guidance on vishing to UK businesses.
Technical controls (call authentication standards like STIR/SHAKEN, which verifies caller identity) help, but staff awareness training is equally important. Staff should know how to verify unexpected callers claiming authority, how to end suspicious calls, and where to report them.
Key Considerations for UK SMEs
- Change all default SIP passwords immediately: Many VoIP systems are deployed with manufacturer default SIP account credentials. Automated scanning tools probe the internet for VoIP systems with default credentials continuously — change them before going live.
- Enable SRTP and TLS on your VoIP platform: If your current VoIP system or SIP trunk does not support encrypted call transport, discuss this with your provider or consider upgrading.
- Set international call limits and blocks: Configure your VoIP platform to alert and block when call spend or volume exceeds defined thresholds, and to block calls to high-risk destinations unless specifically needed.
- Train staff on vishing awareness: Regularly brief all staff on vishing tactics — including that IT support, banks, and HMRC will never ask for passwords or remote access via an unsolicited call.
How AMVIA Can Help
AMVIA configures VoIP security controls as standard on all managed VoIP deployments — including SIP hardening, SRTP/TLS encryption, international call controls, and fraud detection alerting. Security events on VoIP infrastructure are monitored via AmviaIQ. AMVIA also provides vishing awareness content as part of its security awareness training programme for staff. Call 0333 733 8050 to discuss VoIP security for your business.
VoIP Security Controls
Technical measures that protect business VoIP systems from attack.
SIP Authentication Hardening
Strong, unique credentials for SIP accounts — default passwords changed and brute-force protection enabled.
Encrypted Call Transport
SRTP for media encryption and TLS for SIP signalling — prevents eavesdropping on call content.
Toll Fraud Detection
Alerts and automatic blocks when unusual call patterns are detected — international call limits enforced.
SBC and Firewall Controls
Session border controllers and firewall rules restrict VoIP traffic to authorised sources.
VoIP Security Checklist
Essential security controls for business VoIP systems.
Default SIP passwords changed
All SIP account credentials unique and strong — no manufacturer defaults in use.
SRTP and TLS enabled
Call content and signalling encrypted in transit on all VoIP connections.
International call limits configured
Spend and volume thresholds set — automatic block triggers active for anomalous usage.
Session border controller in place
VoIP system not directly internet-facing — SBC provides a security and NAT traversal layer.
Fraud detection monitoring active
Real-time alerts configured for unusual call patterns — not waiting for the monthly invoice.
Staff trained on vishing awareness
Team understands vishing tactics and the correct procedure for handling suspicious calls.
VoIP Security FAQs
The first sign is usually an unexpectedly high phone bill — toll fraud may not be detected until the monthly invoice arrives, by which time significant damage has been done. Proactive monitoring — alerting when call spend or volume exceeds a threshold — catches fraud early. AMVIA configures real-time fraud detection alerts for all managed VoIP clients, triggering automatic blocks if unusual call patterns are detected rather than waiting for the invoice.
Reputable hosted UCaaS providers invest heavily in security infrastructure — encrypted transport, fraud detection, and regular penetration testing are typically included. An on-premise PBX can be equally secure if properly configured and maintained, but it places the security responsibility on the business or its IT provider. For most SMEs, a well-configured hosted platform from a reputable provider offers a comparable or higher security baseline than a self-managed PBX.
SIP scanning is the automated probing of internet-facing IP addresses to identify VoIP systems and test for default or weak credentials. It is carried out continuously by automated tools. Protection involves not exposing your VoIP system's SIP port directly to the internet (use a session border controller instead), using strong non-default SIP passwords, and configuring fail2ban or equivalent brute-force protection. AMVIA deploys session border controllers and applies these controls as standard.
Yes. AMVIA provides vishing awareness training as part of its security awareness programme — covering how vishing works, how to recognise suspicious calls, and the correct response procedure. On the technical side, AMVIA configures caller ID verification and can implement call filtering for known spam and scam numbers as part of a managed VoIP service.
Secure Your Business VoIP System
AMVIA reviews your VoIP security configuration, implements fraud protection and encrypted call transport, and monitors VoIP infrastructure as part of a managed service.
Related Resources
What Is Business VoIP?
How business VoIP works and how it differs from the PSTN it is replacing.
The Complete Cybersecurity Guide
How VoIP security fits into a broader layered security strategy for UK businesses.
Anti-Phishing for UK Businesses
How to protect against phishing and vishing attacks targeting your team.
Business VoIP Solutions
AMVIA's managed VoIP services with security controls included as standard.