What is a VoIP number, and why does it fail verification?

A VoIP number is a phone number that carries calls and texts over the internet instead of over a mobile or landline carrier network. Google Voice, Skype numbers, most Twilio numbers, and the numbers behind many AI agent products are VoIP. The number looks like any other number. The difference is in the database that says who operates it.

How a service tells

When you enter a phone number at signup, many services run it through a line type lookup before they send a code. The lookup returns a type. Twilio Lookup, one of the most widely used, splits VoIP into two:

• fixedVoip. A virtual phone number associated with a physical device, for example Comcast or Vonage.
• nonFixedVoip. A virtual phone number obtained online without requiring a physical device, for example Google Voice or Enflick.

Against those, mobile means a number that generally can receive SMS messages on a carrier network, and landline means a number that generally cannot receive SMS at all.

Why VoIP is treated as risk

VoIP numbers are cheap, fast to create in bulk, and disposable. That makes them the tool of choice for bots, fake accounts and fraud, so services like Google, Microsoft, Discord and Cash App state that they do not accept them for verification, and many others behave the same way without saying so. With a VoIP number, usually one of these happens:

• The service refuses at entry with something like "this number can't be used for verification."
• The code is accepted for sending but never arrives, dropped silently in transit.
• You pass once, then get flagged and locked out later.

Why this matters for AI agents

An agent that needs its own phone number, to sign up for services, receive one-time codes, or talk to people, usually gets that number from a provider. Many of those numbers are VoIP, which means the agent hits the same walls a bot does. numberRank's directory grades agent phone providers by the line type of the numbers they publish and provision.

Check your own number

The VoIP checker runs any number through Twilio Lookup for free, no signup, and shows the line type, carrier and a 0 to 100 score. Developers can call the same check as an API:

curl "https://numberrank.dev/api/check?number=+15551234567&format=text"

One honest caveat

Line type is not a single source of truth. Databases disagree with each other on the same number, and even a real carrier number can be blocked if its range has a bad reputation. A lookup is a strong signal, not a guarantee.