How a VoIP Number Works
The FCC describes VoIP as communicating using internet protocol rather than traditional analog systems. Your voice is converted into a digital data stream and sent as packets over the internet, rather than as a continuous analog electrical signal over copper wire. Some VoIP services only reach users of the same service, while others can call any phone number.
Two types of VoIP service
Interconnected VoIP enables real-time two-way voice communication, requires broadband and IP-compatible equipment, and generally lets users place calls to and receive calls from the public switched telephone network (PSTN). Interconnected VoIP providers must also follow Local Number Portability rules.
Non-interconnected VoIP also enables real-time voice over IP to or from the user’s location and needs IP-compatible equipment, but it is not interconnected VoIP. A service that only reaches users of the same platform falls on this side of the line, and it cannot receive calls or codes from ordinary phone numbers.
| Feature | Interconnected VoIP | Non-interconnected VoIP |
|---|---|---|
| Real-time voice over IP | Yes | Yes |
| IP-compatible equipment needed | Yes | Yes |
| Broadband needed | Yes | Not part of the definition |
| Calls to and from the public phone network | Generally yes, both ways | Not required; some services reach only their own users |
Fixed and non-fixed VoIP
For emergency calling, a device counts as fixed if the user cannot readily move it, such as a wired phone, and non-fixed if it can readily be used at multiple locations. That split also covers interconnected VoIP. The difference is explained in more detail on the page about non-fixed VoIP numbers.
How VoIP providers get numbers
Since 2015, interconnected VoIP providers have been able to apply for telephone numbers directly from the Numbering Administrators; before that, only carriers could. Numbers also reach VoIP services through carrier partners and resellers. On its face, a VoIP number therefore looks like any other phone number.
Number porting and the LRN
As the FCC’s guide to number porting explains, numbers can move between wireline, wireless, and IP providers serving the same geographic area. For a ported number, the portability database returns a Location Routing Number (LRN) — a 10-digit identifier that tells the network which switch now serves that number. Because calls are routed by that identifier, they still reach the right provider after the number has moved. What matters for sign-ups is how carriers classify the line now, and a port to a different kind of provider can change that classification.
Why Apps Often Refuse VoIP Numbers at Sign-Up
Many apps confirm a new account by sending a one-time code by SMS or voice call to the number you enter. That step is where the line type behind a number comes into play. The standards history below explains where one concern came from: a 2017 NIST text, since superseded, said VoIP does not prove possession of a specific device.
What the standards say
Out-of-band authentication means confirming a sign-in through a separate channel, such as a code sent to a phone. The 2017 edition of NIST SP 800-63B stated that methods that do not prove possession of a specific device, such as voice-over-IP (VOIP) or email, must not be used for out-of-band authentication. That version was superseded on August 1, 2025.
Its replacement, NIST SP 800-63B-4, takes a different approach. It labels SMS and voice authentication “restricted” rather than prohibited and tells verifiers to weigh risk signals such as device swap and number porting. Device swap means the number has moved to a different device; number porting means it has moved to a different provider. It contains no explicit VoIP ban.
The practical outcome
A number that carriers classify as VoIP or online is more likely to be refused at sign-up. Virtual numbers are not ruled out, though: one that carriers record as a mobile line passes, so the classification matters more than the label a provider puts on the number. Numbers shared through public receive-SMS sites are often refused as well.
That is why it helps to understand the difference between a non-VoIP number and a number classified as VoIP or online before you pick one for sign-ups.
Common mistake: Assuming that every virtual number will be refused. Virtual is a commercial term, not a line type, and the line type is what decides the outcome.
Choosing and Checking a Number: A Practical Guide
Check the line type first. Before you use a number, find out how carriers classify its line: mobile (non-VoIP) or VoIP/online. If it shows as VoIP or online, expect a higher chance of refusal at sign-up. The guide on how to look up a number’s line type covers this step.
Prefer numbers classified as mobile. Every service accepts non-VoIP mobile lines issued in the US or Australia. A virtual number belongs in this group too when its carrier lists it as mobile rather than as a VoIP or online number — the classification is what matters, not the label the provider uses in its marketing.
Avoid shared public receive-SMS numbers. Sites that publish a phone number for anyone to use show its incoming messages to every visitor, so the number is never yours alone, and apps often refuse numbers used this way.
Account for porting history. If a number has been ported to an IP provider, that provider now serves it, so its line type may no longer be what it was when the number was issued. NIST SP 800-63B-4 also names number porting as one of the risk signals verifiers should weigh.
Read a refusal as a line-type question. When an app refuses a number at sign-up, check its line classification before anything else. Requirements differ by service, so the app’s own help pages are the place to confirm what it accepts.
What to choose
If you need a number that apps accept, the straightforward choice is a US or Australian non-VoIP mobile number; a virtual one comes second, and only once its mobile classification is confirmed. If cost matters, the page on free non-VoIP numbers explains why genuine ones are rare.
Line type comparison
| Criterion | Non-VoIP Mobile | VoIP / Virtual (classified VoIP) | Shared Public Receive-SMS |
|---|---|---|---|
| Carrier classification | Mobile | VoIP or online | Varies |
| Accepted at sign-up | Yes, with all services (US/AU) | Often refused | Often refused |
| Can be ported | Yes | Yes (interconnected VoIP) | Depends on provider |
| Risk level for verification | Low | Medium to high | High |
Common Misconceptions About VoIP Numbers
Myth: an area code shows whether a number is VoIP. Area codes do not reveal line type. A mobile number and a VoIP number can share the same area code. Line type depends on how carriers classify the number, not on its area code.
Myth: NIST still bans VoIP for authentication. The 2017 SP 800-63B text that explicitly named VoIP was superseded on August 1, 2025. SP 800-63B-4, which replaced it, asks verifiers to weigh risk signals and contains no explicit VoIP ban.
Myth: every VoIP number is refused everywhere. Classification matters more than origin. A number that carriers classify as a mobile line can pass verification even if a VoIP provider originally issued it.
Myth: VoIP numbers cannot be ported. Interconnected VoIP providers must follow Local Number Portability rules. Numbers can move between wireline, wireless, and IP providers in the same geographic area.
Genuine limitation: Some VoIP services only reach users of the same service. An account on that kind of service cannot receive calls or codes from ordinary phone numbers, so it is no use for SMS verification.
Frequently Asked Questions
What does a VoIP number mean compared to a regular phone number? A regular phone number is tied to a carrier line — a SIM card or eSIM on a mobile network, or a copper-wire landline. A VoIP number routes calls and sometimes SMS over the internet instead. The number format looks identical; the difference is in how the underlying network handles the call and how carriers classify the line.
Can a VoIP number receive SMS verification codes? It depends on the service sending the code and on how carriers classify the number. An online/VoIP number is often refused at sign-up, and a service that only reaches users of the same platform cannot receive codes from ordinary phone numbers at all. If carriers list a number as mobile (non-VoIP), it passes even when it is virtual.
What is the difference between a VoIP number and a virtual number? The terms overlap but are not identical. VoIP number describes the technology: voice is transmitted over IP. Virtual number is a broader commercial term for any number not tied to a fixed physical line — it includes VoIP numbers but can also describe numbers that forward incoming calls to a mobile line. For sign-ups, what counts is how carriers classify the line, not which of the two labels a provider uses.
Can I keep my existing number if I switch to a VoIP service? Generally, yes. Numbers can be ported between wireline, wireless, and IP providers serving the same area. After the port, the portability database returns an LRN for the VoIP provider’s switch, since that switch now serves the number.
Why does my number show as VoIP when I think it is a mobile number? Line type follows how carriers classify the number, not how you use it. A number that has been ported to an IP provider is now served by that provider, and numbers can also reach VoIP services through carrier partners or resellers. For an app’s own rules, check its official help page.