Why banks reject Google Voice (VoIP vs. mobile numbers)
"This phone number can't be used for verification." If you've seen that message, the problem usually isn't the number. It's the line type.
You type your number into your bank's sign-up form, press continue, and get an error. The number is real. It receives texts from friends without any trouble. But the bank won't send it a code. If the number came from Google Voice or another internet calling app, you've run into one of the most common walls in online banking: the line-type check.
What "line type" means
Every phone number in the U.S. belongs to a carrier, and carrier databases record what kind of line it is. The main categories are mobile (a wireless line, like the one in your phone), landline, and VoIP (a number that runs over the internet).
VoIP itself is split in two. Fixed VoIP is tied to a physical address, like a home phone from a cable company. Non-fixed VoIP can be created online by anyone, from anywhere, often for free. Google Voice and most texting apps fall into this second group.
Anyone can look this up. Banks, payment apps and messaging services use lookup services that return the line type, the carrier and sometimes more, in a fraction of a second.
Why banks care
Fraud. A bank uses your phone number as proof that you are a real, reachable person. Non-fixed VoIP numbers are cheap and quick to create in bulk, which makes them attractive for fake accounts. Blocking the whole category is a blunt but simple way to cut that risk.
Identity. Mobile numbers are usually sold by carriers with some account behind them. Banks treat that as a weak but useful signal that the number belongs to a specific person.
Recycling. Free numbers can be reclaimed after a period without use and handed to someone else. If your bank's codes go to a recycled number, a stranger receives them.
None of this means your VoIP number is unsafe. It means the bank can't tell your number apart from the risky ones, so it turns them all away.
Why Google Voice in particular
Google Voice is the best-known free U.S. number, so it's the one people try first. Its numbers are listed as VoIP. Some services accept them, many don't, and a service that accepts them today can stop tomorrow. For people outside the U.S., setting up a personal Google Voice number is usually not possible in the first place.
How to check your own number
Search for a free "phone number line type lookup" and enter your number with the +1 country code. If the result says VoIP, expect trouble with banks and payment apps. If it says mobile or wireless, you're in a much better position.
What to do if your bank rejects your number
Use a mobile number. A carrier SIM works if you're in the U.S. If you're not, a mobile-registered number you use in the browser gives you the same line type without the SIM.
Ask about other methods. Some banks let you switch to an authenticator app after sign-up. Turn it on as a backup either way.
Change numbers before you lose access. If your account still uses an old number, update it while you can still receive codes there.
Why "usually" and not "always"
Line type is the most common check, but not the only one. Some services also look at the carrier name, how long the number has existed, or whether it belongs to a traditional carrier at all. That's why we say mobile numbers are usually accepted, and why we publish dated test results for services like Mercury, Stripe and Wise.