How to Detect VoIP Numbers (and Why It Matters for Fraud)

A phone number used to be a meaningful cost and commitment. VoIP changed that. Today anyone can provision a fresh virtual number in seconds, for free, from dozens of apps and providers. That convenience makes VoIP one of the most important signals in phone-based fraud detection, because the qualities that make VoIP attractive to users make it invaluable to fraudsters.

Why VoIP is a fraud signal

VoIP numbers are effectively disposable. A fraudster can burn through hundreds of them to bypass phone verification, create fake accounts, farm signup bonuses, or evade a ban. Unlike a mobile number tied to a SIM and a carrier contract, a VoIP number carries little accountability and can be discarded the moment it's flagged.

• Bypassing OTP: a virtual number receives the SMS code just like a real phone.
• Multi-accounting: unlimited cheap numbers mean unlimited 'verified' accounts.
• Ban evasion: a new number sidesteps blocks tied to the old one.

How VoIP detection works

Detecting VoIP means looking up the number's carrier and line type. Every number is allocated to a carrier and a numbering-plan block; those allocations reveal whether a number is mobile, landline, or VoIP, and often whether it's routed through a CLEC (a competitive carrier commonly used by VoIP providers). A number that resolves to a VoIP carrier or line type is your flag.

curl -X POST https://api.riskunified.com/phone_risk \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"phone": "2025550142"}'

The Phone Risk API returns line_type, a voip boolean, the operating carrier, prefix_type, and a prepaid flag, plus a rolled-up risk level so you can act immediately.

{
  "phone_features": {
    "carrier": "BANDWIDTH.COM CLEC, LLC",
    "line_type": "voip",
    "voip": true,
    "prepaid": false
  },
  "risk": { "level": "medium", "signals": ["voip"] }
}

Acting on VoIP without punishing real users

Plenty of legitimate people use VoIP as their primary line, so a blanket block is a mistake. The stronger play is to raise friction proportionally: allow low-risk actions, require an additional factor for medium risk, and reserve hard blocks for VoIP combined with other red flags like a disposable email or a datacenter IP.