Last year, a 40-person accounting firm noticed something odd on their monthly phone bill: a cluster of international calls to premium-rate numbers in Eastern Europe, racked up over a single weekend when the office was empty. Their VoIP system had been compromised through a default admin password on a SIP trunk nobody remembered provisioning. The damage was $14,000 in toll fraud charges and three days of downtime while the provider locked and rebuilt the account.
That scenario is preventable. A VoIP security audit finds exactly these kinds of gaps — weak credentials, forgotten endpoints, unencrypted traffic — before an attacker does. This guide walks you through how to run one, from scoping the work to fixing what you find.
An audit that misses devices misses vulnerabilities. Start by building a complete inventory of your VoIP infrastructure and defining what you want the audit to accomplish.
List every component that touches voice traffic:
Check procurement records, network scans, and DHCP logs to catch devices that were deployed but never documented. Shadow IT endpoints — a softphone someone installed on a personal laptop, a conference bridge spun up for a one-time project — are common blind spots.
Vague goals produce vague results. Define measurable targets:
Before testing anything, gather the paperwork: network topology diagrams, SIP server and firewall configurations, access control policies, encryption certificates with expiration dates, patch history logs, and incident response procedures. This baseline lets you spot unauthorized changes and configuration drift during the audit.
This is the hands-on phase. Work through each layer methodically so nothing gets skipped.
Review how your VoIP network is isolated from general traffic:
A flat network where VoIP shares broadcast domains with printers, laptops, and IoT devices gives attackers a direct path from a compromised workstation to your phone system.
Weak credentials caused that accounting firm’s toll fraud incident, and they remain the single most exploited VoIP vulnerability:
Unencrypted VoIP traffic can be captured and played back with free tools like Wireshark. Verify these protections are active, not just configured:
Use a protocol analyzer to capture a sample of live traffic and confirm encryption is functioning end to end, not just at the first hop.
Automated scanning catches known vulnerabilities. Manual testing catches the ones scanners miss.
Scan every VoIP server, gateway, and SBC with a scanner that includes SIP-specific checks. Flag outdated firmware, known CVEs, and misconfigured services. Cross-reference findings against your asset inventory to confirm full coverage.
Go beyond generic network pen testing. VoIP-targeted tests should include:
Document each finding with reproduction steps, severity rating, and a specific remediation action.
Set up or verify continuous monitoring that catches threats between audits:
Take your audit findings and fix them, starting with the highest-severity issues.
Outdated firmware is a prime target. Build a patch schedule that covers SIP servers, SBCs, IP phones, and gateways. Test patches in a staging environment before deploying to production, and track compliance with a simple spreadsheet or your configuration management tool.
When a breach happens, centralized logging is the difference between identifying the root cause in hours versus weeks.
A single audit is a snapshot. Threats change, employees rotate, and new devices get added. Build a recurring schedule:
Assign clear ownership for each task, track completion, and review audit trends over time to measure whether your security posture is actually improving.
These mistakes undermine even well-planned audits:
Run automated vulnerability scans quarterly and a full-scope audit annually. If your organization handles regulated data (healthcare, financial services), your compliance framework may require more frequent assessments. Any major infrastructure change — a new SIP trunk provider, a phone system migration, a network redesign — should also trigger an audit.
Weak or default credentials. Many VoIP breaches start with an attacker brute-forcing a SIP registration password or logging into an admin interface using the factory default username and password. Enforcing strong, unique passwords and multi-factor authentication eliminates the majority of these attacks.
You can handle routine checks in-house — vulnerability scans, credential reviews, configuration audits. For penetration testing and full-scope audits, an external firm brings fresh eyes and specialized VoIP testing tools. A hybrid approach (in-house quarterly scans, external annual audit) balances cost and thoroughness.
Use a packet capture tool like Wireshark on a network segment carrying voice traffic. If you see SIP traffic on port 5061 (TLS) rather than 5060 (plaintext), and media streams show as SRTP rather than RTP, your encryption is active. Your SBC or IP-PBX admin interface may also display encryption status per call or per trunk.
Isolate the affected component immediately if the vulnerability is actively exploitable. Apply the vendor patch or configuration fix, then re-test to confirm the issue is resolved. Document the finding, the remediation steps, and the verification result. Update your audit checklist so future audits specifically check for recurrence.
A security audit is only as strong as the infrastructure underneath it. Running VoIP on unreliable or unmanaged network services creates vulnerabilities that no audit can fully compensate for.
1stel provides the secure, business-grade foundation your phone system needs:
Contact 1stel to discuss how secure, managed telecom infrastructure can simplify your next VoIP security audit.