What Are the Biggest VoIP Security Threats and How Do You Stop Them?

Business phone systems have moved to the cloud, and for good reason. VoIP (Voice over Internet Protocol) cuts costs, simplifies management, and enables remote work in ways traditional phone lines never could. But running voice traffic over your data network introduces security risks that many businesses overlook until something goes wrong.

VoIP-specific attacks cost businesses billions of dollars annually. The Communications Fraud Control Association (CFCA) estimated global telecom fraud losses at $38.95 billion in 2023, with toll fraud and traffic pumping among the top contributors. Small and mid-sized businesses are frequent targets precisely because attackers assume they lack dedicated security resources.

Here’s what you need to know about the most pressing VoIP threats — and what you can actually do about each one.

What Is VoIP Toll Fraud and Why Is It So Expensive?

Toll fraud is the single most costly VoIP security threat. It occurs when an attacker gains unauthorized access to your phone system and places long-distance or premium-rate calls at your expense. Attackers typically compromise a SIP trunk, an IP phone’s credentials, or a voicemail system configured to allow outbound dialing, then route thousands of calls to premium-rate numbers they control — often in international destinations where per-minute charges are highest.

A compromised system can rack up tens of thousands of dollars in fraudulent charges over a single weekend. Because carriers bill the account holder regardless of who placed the calls, businesses are usually on the hook for the full amount.

How Do You Prevent Toll Fraud?

Preventing toll fraud starts with basic access controls that too many organizations skip:

• Disable international dialing on every extension that doesn’t explicitly need it. Most employees never call international numbers, so there’s no reason to leave that door open.
• Set concurrent call limits on SIP trunks. If your office normally handles 20 simultaneous calls, a sudden spike to 200 should trigger an automatic block.
• Use strong, unique passwords for every SIP endpoint. Default credentials are the first thing attackers try, and brute-forcing weak passwords takes minutes with automated tools.
• Enable call detail record (CDR) monitoring with alerts for unusual patterns — calls to high-risk country codes, calls outside business hours, or abnormal call volumes.
• Work with your provider to set spending caps and get real-time fraud alerts. Reputable VoIP providers offer these controls as standard features.

Can Attackers Eavesdrop on VoIP Calls?

Yes. Because VoIP calls travel as data packets over your network, anyone with access to that network can potentially intercept and listen to conversations. This is called packet sniffing or call interception, and it’s a real concern for businesses that handle sensitive information — financial data, legal discussions, healthcare records, or proprietary business strategy.

Unencrypted SIP signaling and RTP (Real-time Transport Protocol) media streams are particularly vulnerable. An attacker on the same network segment, or anywhere along the packet’s route, can capture the traffic and reconstruct the audio using freely available tools like Wireshark.

How Do You Encrypt VoIP Traffic?

The fix is straightforward: encrypt everything.

• TLS (Transport Layer Security) encrypts SIP signaling, protecting call setup data including caller identity, numbers dialed, and authentication credentials.
• SRTP (Secure Real-time Transport Protocol) encrypts the actual voice media, making intercepted packets unintelligible.
• VPN tunnels for remote workers add another layer, ensuring VoIP traffic between home offices and your business network stays encrypted end-to-end.

Confirm that your VoIP provider supports TLS and SRTP — and that both are actually enabled. Having the capability and having it turned on are two different things. At COMNEXIA, we configure encryption by default on every business phone system deployment because leaving it optional means it gets forgotten.

What Is Vishing and How Does It Target Businesses?

Vishing — voice phishing — uses phone calls instead of emails to trick employees into revealing sensitive information. Attackers spoof caller ID to impersonate banks, vendors, IT support, or even company executives, then pressure employees into sharing passwords, transferring funds, or granting system access.

Vishing has surged alongside the rise of AI-generated voice synthesis. Attackers can now clone a person’s voice from a few seconds of publicly available audio, making impersonation calls disturbingly convincing. The FBI’s Internet Crime Complaint Center (IC3) has flagged deepfake audio as a growing threat vector in business fraud.

How Do You Protect Against Vishing Attacks?

Technical controls help, but vishing is fundamentally a human-targeted attack, so training is essential:

• Security awareness training should include vishing scenarios — not just email phishing. Employees need to practice recognizing pressure tactics and pretexting over the phone.
• Verification procedures for sensitive requests. Any call requesting wire transfers, password resets, or system access changes should require callback verification through a known number — never the number the caller provides.
• STIR/SHAKEN implementation helps validate caller ID authenticity. The FCC mandated STIR/SHAKEN for major carriers in 2021, and your VoIP provider should support it.
• Call recording and logging create an audit trail that deters fraud and aids investigation when incidents occur.

What Is a Denial-of-Service Attack on a Phone System?

A Denial-of-Service (DoS) attack against VoIP infrastructure floods your phone system with traffic — typically malformed SIP requests, excessive registration attempts, or high-volume junk calls — until legitimate calls can’t get through. For businesses that depend on phone availability (sales teams, support desks, medical offices), even a few hours of downtime translates directly to lost revenue and damaged customer relationships.

Distributed Denial-of-Service (DDoS) attacks are worse, using botnets to generate traffic from thousands of sources simultaneously, making simple IP blocking ineffective.

How Do You Defend Against VoIP DoS Attacks?

• Session Border Controllers (SBCs) sit at the edge of your network and filter VoIP traffic, blocking malformed packets and rate-limiting connection attempts before they reach your phone system.
• Intrusion detection and prevention systems (IDS/IPS) tuned for SIP traffic can identify and block attack patterns in real time.
• Network segmentation isolates voice traffic on its own VLAN, preventing attacks on your data network from spilling over into phone services — and vice versa.
• Geo-IP filtering blocks SIP traffic from countries where you don’t do business. If you have no customers or offices in certain regions, there’s no reason to accept SIP connections from those IP ranges.
• Redundancy and failover ensure that if one system is overwhelmed, calls route to a backup automatically. Cloud-based VoIP systems typically handle this better than on-premises PBX hardware.

What About SIP Registration Hijacking?

SIP registration hijacking occurs when an attacker intercepts or forges SIP REGISTER messages to redirect incoming calls to their own device. The legitimate user’s phone stops ringing, and the attacker receives all their calls — potentially capturing sensitive business conversations, customer information, or authentication codes delivered by phone.

This attack exploits weaknesses in SIP authentication, particularly when digest authentication uses weak passwords or when registration messages travel unencrypted.

How Do You Prevent SIP Hijacking?

• Mutual TLS authentication between endpoints and your SIP server ensures both sides verify identity before establishing a session.
• Short registration expiration timers force endpoints to re-register frequently, reducing the window an attacker has to exploit a hijacked registration.
• IP address binding restricts each SIP account to register only from known IP addresses or subnets.
• Monitor registration events for anomalies — multiple registrations from different IPs, registrations from unexpected geographic locations, or sudden de-registrations of active extensions.

How Should Businesses Approach VoIP Security Overall?

VoIP security isn’t a single product you buy — it’s a combination of configuration, monitoring, and ongoing maintenance. The most effective approach treats voice infrastructure with the same rigor as any other network service:

1. Audit your current setup. Document every SIP trunk, every endpoint, every integration point. You can’t secure what you don’t know about.
2. Encrypt everything. TLS for signaling, SRTP for media, VPNs for remote access. No exceptions.
3. Harden credentials. Strong passwords, multi-factor authentication where supported, and regular credential rotation.
4. Segment your network. Voice traffic on dedicated VLANs with QoS policies that also serve as security boundaries.
5. Monitor continuously. CDR analysis, failed authentication alerts, traffic anomaly detection. Automated monitoring catches what humans miss.
6. Keep firmware current. IP phones, SBCs, and PBX software all need regular updates. Unpatched systems are low-hanging fruit for attackers.
7. Partner with experts. VoIP security requires knowledge of both telecommunications and cybersecurity. Working with a provider that understands both domains — like COMNEXIA, which has supported business communications across the Atlanta metro for over 35 years — reduces the burden on your internal team.

Frequently Asked Questions

What is the most common VoIP security threat? Toll fraud is the most common and most costly VoIP security threat. Attackers compromise phone system credentials to place unauthorized international or premium-rate calls, leaving the business responsible for charges that can reach tens of thousands of dollars in a single incident.

Are VoIP calls less secure than traditional landlines? Not inherently, but they face different risks. Traditional landlines required physical wiretapping; VoIP calls can be intercepted remotely if unencrypted. However, properly configured VoIP with TLS and SRTP encryption is actually more secure than an analog phone line, which has no encryption at all.

Does my business need a Session Border Controller? Any business running SIP trunks should strongly consider an SBC. It acts as a firewall specifically for voice traffic, filtering malicious SIP messages, preventing unauthorized access, and protecting against DoS attacks. For businesses with on-premises phone systems, an SBC is essentially mandatory.

How do I know if my VoIP system has been compromised? Warning signs include unexpected spikes in call volume (especially international calls), calls placed outside business hours, unfamiliar numbers in call logs, unusually high phone bills, and users reporting that their extensions aren’t ringing. Regular CDR review and automated alerting catch most compromises early.

Can small businesses afford proper VoIP security? Yes. Many VoIP security measures — strong passwords, disabling unused features, enabling encryption, setting call restrictions — cost nothing beyond the time to configure them. The real expense comes from not securing your system: a single toll fraud incident can cost more than years of proactive security investment.