The Top 5 VoIP Security Risks and How to Avoid Them

A controller at a 40-person manufacturing company logged in on Monday morning to find $12,000 in international calls billed over the weekend — calls no one at the company made. Attackers had exploited a default password on an unused VoIP extension, routed hundreds of calls to premium-rate numbers overseas, and disappeared before anyone noticed.

This is toll fraud, and it is just one of five security risks that come with running your phone system over the internet. VoIP gives your business flexibility, lower costs, and powerful integrations with tools like CRMs and video conferencing. But because VoIP traffic runs on the same network as your data, it inherits every vulnerability that network carries — plus a few that are unique to voice.

Here are the five most common VoIP threats, how they work, and exactly what you can do to stop them.

1. Vishing: Phone-Based Phishing That Exploits Caller ID Trust

Vishing is phishing delivered by voice. An attacker calls an employee, spoofs the caller ID to display an internal extension or a known vendor’s number, and asks for credentials, payment details, or remote access. Because people inherently trust phone calls more than emails, vishing attacks succeed more often than you might expect.

A typical scenario: someone posing as your IT department calls an employee and says they need the employee’s VoIP portal password to “fix a system issue.” The employee complies, and the attacker now owns that account.

How to stop it:

• Require multi-factor authentication (MFA) on every VoIP account. A stolen password alone will not grant access.
• Establish a verification policy. Employees should hang up and call back on a known-good number before sharing any credentials or approving any changes.
• Deploy STIR/SHAKEN caller ID authentication. This protocol verifies that the calling number has not been spoofed, making fraudulent calls easier to identify.
• Monitor login activity. Flag repeated failed logins, logins from unusual locations, or access outside business hours.

2. Denial of Service Attacks That Shut Down Your Phones

A Denial of Service (DoS) or Distributed Denial of Service (DDoS) attack floods your VoIP servers with fake traffic — thousands of SIP requests per second — until legitimate calls cannot get through. Your phones go silent, customers cannot reach you, and internal communication stops.

For businesses that depend on phone-based sales, support, or dispatch, even 30 minutes of downtime translates directly into lost revenue.

How to stop it:

• Segment your network. Run VoIP traffic on a dedicated VLAN, isolated from your general data network. This limits the blast radius of any attack.
• Deploy intrusion detection and prevention systems (IDPS). These tools spot abnormal traffic spikes and block malicious requests before they reach your VoIP servers.
• Set rate limits on SIP ports. Cap how many requests a single IP address can make per second.
• Use redundant internet connections. If one line is saturated by attack traffic, a second connection keeps your phones running.
• Choose an ISP that offers built-in DDoS mitigation. Providers of business internet services with network-level DDoS protection can absorb attack traffic before it ever reaches your office.

3. Toll Fraud: Unauthorized Calls Billed to Your Account

Toll fraud — the scenario from the opening of this article — happens when attackers gain access to your VoIP system and route calls through it, typically to international or premium-rate numbers they control. The calls generate revenue for the attacker, and the charges land on your bill.

The most common entry point is a weak or default password on a VoIP extension, voicemail box, or admin portal.

How to stop it:

• Enforce strong, unique passwords on every extension and admin account. Eliminate default credentials the day you deploy.
• Restrict outbound calling permissions. Block international and premium-rate dialing on extensions that do not need it. Require manager approval to unlock those destinations.
• Set spend alerts and call-volume thresholds. Get notified immediately if call costs or volumes exceed normal patterns, especially outside business hours.
• Disable unused extensions and accounts. Dormant accounts with weak passwords are the easiest targets.
• Audit call detail records weekly. Look for calls to unfamiliar country codes, calls placed at unusual hours, or sudden spikes in volume.

A business telephone provider that actively monitors call activity and flags anomalies adds another layer of protection beyond what you manage internally.

4. Malware and Eavesdropping on Unencrypted Calls

VoIP systems that transmit voice data without encryption are vulnerable to interception. An attacker with access to your network can capture VoIP packets and reconstruct entire conversations. One well-known technique, called VOMIT (Voice over Misconfigured Internet Telephones), specifically targets misconfigured VoIP devices to extract unencrypted audio.

Beyond eavesdropping, malware installed on VoIP servers, gateways, or IP phones can harvest credentials, redirect calls, or open backdoors into your broader network.

How to stop it:

• Encrypt all VoIP traffic. Use TLS for signaling and SRTP for media streams. Without encryption, intercepted packets are trivially readable.
• Keep firmware and software current. Apply vendor patches as soon as they are released. Attackers actively scan for known VoIP vulnerabilities.
• Run endpoint protection on VoIP servers. Anti-malware tools and file-integrity monitoring catch threats that slip past network defenses.
• Audit device configurations regularly. Misconfigured devices — open ports, disabled encryption, default settings — are the most common weak point.

Platforms like 1stConnect centralize monitoring across your communication infrastructure, making it easier to verify that encryption is active and configurations have not drifted.

5. Caller ID Spoofing and Man-in-the-Middle Attacks

Spoofing lets an attacker disguise their phone number so it appears to come from a trusted source — your CEO’s extension, your bank, or a vendor. Combined with social engineering, spoofed calls can trick employees into authorizing wire transfers, sharing sensitive data, or granting system access.

In a man-in-the-middle attack, the attacker inserts themselves between two parties on a call, intercepting or altering the conversation without either side knowing.

How to stop it:

• Implement STIR/SHAKEN across your phone system. This is the single most effective measure against spoofed caller IDs.
• Encrypt all call signaling and media. Encryption prevents man-in-the-middle attacks by making intercepted data unreadable and tamper-evident.
• Train employees to verify before trusting. Any request for money, credentials, or access that comes by phone should be confirmed through a separate channel.
• Review call logs for spoofing patterns. Repeated calls from numbers that do not match their claimed origin are a red flag.

Frequently Asked Questions

What is the single most important step to secure a VoIP system? Encrypt everything. Enable TLS for signaling and SRTP for media on every device and trunk. Encryption blocks eavesdropping, prevents packet reconstruction attacks like VOMIT, and stops man-in-the-middle interception. Without it, every other security measure is working around an open door.

How do I know if my business is a target for toll fraud? Every business with a VoIP system is a potential target. Attackers use automated scanning tools to find systems with weak passwords or default credentials. Small and mid-sized businesses are hit frequently because they are less likely to have dedicated security monitoring. If you have any extensions with default passwords, unused accounts still active, or no restrictions on international dialing, your risk is elevated.

Can VoIP be as secure as a traditional landline phone system? Yes — and in many cases more secure. Traditional landlines were not encrypted, so anyone with physical access to the copper line could tap a call. A properly configured VoIP system with TLS/SRTP encryption, MFA, network segmentation, and active monitoring provides stronger protection than a legacy phone system ever did.

What should I look for in a VoIP provider’s security practices? Ask whether the provider encrypts all traffic by default, offers built-in DDoS protection, supports STIR/SHAKEN for caller ID authentication, monitors for toll fraud in real time, and provides regular security updates. A provider that cannot answer these questions clearly is not one you want handling your business communications.

How often should we train employees on VoIP security? At minimum, run training annually with a refresher whenever a new threat emerges. Vishing tactics evolve quickly, and a one-time training session loses effectiveness within months. Short, scenario-based exercises — like simulated vishing calls — are more effective than lengthy presentations.

Protect Your Business Communications

VoIP security is not a set-it-and-forget-it task. Attackers adapt, and your defenses need to keep pace. The five risks outlined here — vishing, DoS attacks, toll fraud, malware and eavesdropping, and spoofing — are preventable when you combine encryption, MFA, network segmentation, employee training, and active monitoring.

If you are evaluating your current phone system’s security posture or considering a move to VoIP, 1stel can help. We provide business telephone services with built-in security controls, business internet with DDoS protection, and 1stConnect for unified communications that keeps your voice, video, and messaging secure under one platform. Contact 1stel today to discuss how to lock down your business communications.