What is a non-VoIP number, and why won't some services text one?

How services tell the difference

Before a business texts you a code, it can pay for a lookup of your number's line type. Twilio's Line Type Intelligence is one widely used lookup. It returns types such as mobile, landline, fixedVoip and nonFixedVoip. As examples of nonFixedVoip, its documentation gives Google Voice and Enflick, the company behind TextNow.

Fixed vs non-fixed VoIP

Both are internet numbers. The difference is the hardware. Twilio defines a fixed VoIP number as “a virtual phone number associated with a physical device. For example, Comcast or Vonage.” A non-fixed VoIP number is “a virtual phone number obtained online without requiring a physical device. For example, Google Voice or Enflick” (Twilio).

Non-fixed numbers take a minute to create and nothing to give up, which is why services that filter VoIP treat them as the riskier kind.

“Non-VoIP” isn't a type. It's the absence of a flag

Nobody issues a non-VoIP number. No regulator defines the term, no carrier prints it on a bill, and no database has a field for it. It is a word the market invented for a number that a commercial lookup does not label as VoIP. That sounds like hair-splitting until you need to argue with a rejection, at which point it is the whole problem: there is no authority you can appeal to, because there is no authority that issued the label.

What a lookup actually returns is one of a fixed set of values. Twilio's Line Type Intelligence publishes the full list, and only two of them are the VoIP kinds people talk about (Twilio):

Read the last row again. A number can be perfectly real, perfectly yours, and come back as unknown. Twilio also warns that “Carrier data isn't available for phone number types: personal, tollFree, premium, sharedCost, uan, voicemail, pager, or unknown.” A service that treats anything other than mobile as suspicious will turn those numbers away too, and its error message will still say the number is not supported.

How to check which kind a number is

There is no public register to look a number up in. The check a business runs is a paid lookup against carrier data, and the free “is this VoIP?” pages are working from less than that. Three things you can do instead:

1. 01Ask whoever issued it. The company that gave you the number knows what it is. A carrier will say mobile. An app that handed you a number in a minute, with no SIM and no credit check, gave you a non-fixed VoIP number.
2. 02Run the lookup the businesses run. Twilio's Line Type Intelligence answers mobile, landline, fixedVoip or nonFixedVoip for a number (Twilio). It is a paid API rather than a web page, which is why the answer is hard to get for free.
3. 03Read the refusal. A service that has already turned your number down has run the check for you. That is usually the moment people start looking the term up.

Treat a free lookup's answer as a guess. Numbers move between carriers, and a lookup that has not caught up gives you the old answer with the same confidence as the right one.

Why a lookup can call a real mobile line VoIP

This is the part that is almost never written down, and it is the reason people end up shouting at a support agent. The check is not a fact about your number. It is a database's opinion about your number, and databases are wrong in specific, predictable ways. One company says so out loud about its own detection:

• Steam Support, on its own VoIP detection

  “Select carriers use VoIP technology in their cellular plans and we are unable to support these.”

  https://help.steampowered.com/en/faqs/view/7EFD-3CAE-64D3-1C31

That is a company with a hard VoIP ban conceding that real cellular plans get caught by it. Four things make a real line read wrong, and all four are structural rather than anybody's mistake:

• The number moved carriers. US law guarantees it can. Number portability is “the ability of users of telecommunications services to retain, at the same location, existing telecommunications numbers without impairment of quality, reliability, or convenience when switching from one telecommunications carrier to another” (47 CFR 52.21). Every port is recorded in “a secure and efficient central platform called the Number Portability Administration Center (NPAC)” (iconectiv). A lookup that has your number's old home has the wrong answer, confidently.
• The number came out of a block. Numbers are handed to carriers in batches: pooling separates “the 10,000 numbers in a central office code (NXX)” into “ten sequential blocks of 1,000 numbers each” (47 CFR 52.20). Ask what carrier a number belongs to and the cheapest answer is whoever holds its block. That answer is right until the number moves, and then it quietly isn't.
• It was somebody else's number first. Disconnected numbers are recycled after an aging period: residential numbers “may be aged for no less than 45 days and no more than 90 days”, business numbers up to 365 (47 CFR 52.15). The FCC set the floor deliberately — “We establish a minimum aging period of 45 days, in the middle of the proposed range, for all numbers” (FCC 18-177). Forty-five days is not long for every database in the country to notice that your number has a new owner and possibly a new carrier.
• Your carrier is genuinely built on VoIP. That is the Steam quote above. Some perfectly ordinary cellular plans run voice over IP under the hood, and a detector looking for that signature finds it.

What nobody publishes is how often any of this happens. Twilio's current Line Type Intelligence page states no accuracy figure and no update cadence. Its older v1 documentation is franker about where the data comes from: the carrier name is “often subject to change”, and an empty answer means “The phone number is valid but no information was returned from our data providers” (Twilio). And Twilio's own guide telling businesses to filter VoIP before sending a code carries no warning about false positives at all (Twilio). We will not put a percentage on it, because no first-party source publishes one.

What to do when your own number reads as VoIP

There is no appeals process. No lookup vendor publishes a way for a member of the public to correct a record about their own number, and the service rejecting you is reading someone else's database rather than keeping one. So the moves that work are all sideways.

1. 01Get the rejection in writing, and read the wording. Some services tell you exactly what they decided. Steam says “Our system will notify you when you enter your phone number if we consider it to be a VoIP number” (Steam). “We can't send a code to this number” and “this number is not supported” are different problems.
2. 02Ask for the code by phone call. A voice call often takes a route that the text filter never sees, and plenty of services offer it without advertising it. Verification by automated phone call covers who does.
3. 03Ask your carrier what the line is registered as, and whether it was recently ported or reassigned to you. A number you received in the last few months is the likeliest to still carry someone else's record.
4. 04Try a different service first before you change anything. If one site accepts the number and another doesn't, the number is fine and the second site's filter is strict. If nothing accepts it, the record is the problem.
5. 05Take the phone number out of the loop where you can. An authenticator app, a passkey or a security key sidesteps the entire question, permanently, and most services that insist on SMS still offer one of them somewhere in their settings.
6. 06If you need a second, clean line, get one that is a mobile line to begin with — your own SIM, a prepaid US SIM, or a real mobile line delivered online. Moving to another internet number just changes which database says no.

Be realistic about the ceiling here. Nobody can guarantee a given number reads as mobile at a given company on a given day, and a provider who tells you otherwise is describing a database they do not control. That includes us.

What “non-wireless” means

Some forms and support agents say “non-wireless” instead. It means any number that isn't a cellular line: landlines, fixed VoIP and non-fixed VoIP. A request for a “wireless” number is a request for a mobile line. Venmo's user agreement, for one, asks for “a cellular/wireless telephone number that you own” (Venmo and VoIP numbers).

Twilio's advice to the businesses that use it is to filter before sending: “VoIP numbers are often correlated with bots and don't have the same assurance that you're interacting with a real and unique user” (Twilio).

Why the rules got stricter

Text-message codes are a target for fraud. The FTC reported $330 million lost to text scams in 2022 (FTC). Security guidance moved too. NIST's digital identity guidelines (SP 800-63B, revision 4) treat codes sent over the phone network as a “restricted” authenticator. Verifiers “SHOULD consider risk indicators (e.g., device swap, SIM change, number porting, other abnormal behavior)” (NIST). The previous revision was blunter: methods “that do not prove possession of a specific device, such as voice-over-IP (VOIP) or email, SHALL NOT be used” (NIST).

What internet-number providers say themselves

Companies that sell VoIP numbers are open about the limit in their own help pages:

• Hushed

  “NOT guaranteed to work with third-party verification due to blocks implemented by certain services”

  https://support.hushed.com/hc/en-us/articles/360015707631

• MySudo

  “some services do not support VoIP numbers by design”

  https://support.mysudo.com/hc/en-us/articles/360019981814

• Quo (formerly OpenPhone)

  “In many cases, services that use 2-factor authentication (2FA) don't allow virtual phone numbers to be used during the registration/verification step”

  https://www.quo.com/pricing

• MultiMFA

  “MultiMFA SMS numbers are VoIP numbers. Some platforms reject VoIP numbers”

  https://multimfa.com/shared-sms-verification-number

“Non-VoIP” doesn't mean “accepted everywhere”

A mobile line passes the line-type check. It doesn't exempt you from a service's other rules: who you are, where you are, your account's history. Most services that turn away VoIP accept a real mobile line. No provider can promise that every service accepts every number. Cleat doesn't either.

Your options, honestly

1. 01Your own mobile number. It works. But every account then depends on one SIM, and your personal number goes to every site that asks for one.
2. 02A prepaid US SIM or eSIM. A real mobile line. You need a compatible phone or eSIM slot, and some carriers only activate inside the US: Tello's eSIM must be activated while “physically located within the United States” (Tello).
3. 03An authenticator app or a security key. Stronger than text codes. Use one wherever a service offers it; many banks still insist on SMS.
4. 04A dedicated real mobile line delivered online. That's what Cleat is. A US mobile number that receives texts into a web inbox, email, Telegram or your own tools. One verified owner, no SIM to install. Cleat Line is receive-only; Cleat Full adds calls in the web app.
5. 05Free “receive SMS online” sites. Public, shared, reused numbers. Anyone can read a code sent to them. Don't use them for anything you care about.

Who this isn't for

Cleat lines are for your own accounts and your team's. Every line has a verified owner. Cleat isn't for running lots of numbers, for accounts in someone else's name, or for a number nobody can trace to you. Why we verify every owner.

Keep reading

Google Voice for verification codes · When Google Voice takes your number back · What each service publishes about VoIP · Is TextNow VoIP? · A shared 2FA number for teams · A US number for expats and non-residents · Cleat pricing · Why verified numbers work · Cleat or Quackr · What WhatsApp asks of a number · A second number for dating apps