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Yeah, prepaid phone numbers. I assume it's also likely a US thing as well. If you drop down to the store and buy a TracPhone or Cricket Wireless product or whatever and load it up with a few pre-paid minutes, that'll fail a lot of the identity verification steps happening behind the scenes when you try to create accounts with it. Generally post-paid plans (or "contracts" as they're often called, since we love overloading words) will work for that sort of thing day one.

My number in EU (LTU) is not prepaid, but likely on some bullshit outdated prefix blacklist, so US companies (microsoft and blizzard so far) do not allow to use it for authenticators and such for being a prepaid number. MS simply refused to send the SMS, blizzard explicitly mentioned the prepaid reason.

I think they are referring to VOIP numbers which are blocked from usage for the vast majority of sign ups requiring a phone number