For a moment I was thinking that perhaps this could be the foundation for something to replace Google Voice for some people. Unfortunately I am not sure that most SIP telephone numbers will be usable for things like account verification.
If I get a number via Twillio/$X is the receiver of the call able to tell? I haven't spent a lot of time with SIP and POTS stuff. All my time has been WebRTC and got into SIP for work.
Unfortunately, yes. After I ported my number from at&t to google voice, a lot of services refused to accept it. Requiring SMS 2FA with an non-VOIP number seems to be a common anti-spam measure these days. It's often required for new accounts
To get a little deeper into it, Twilio (in Canada at least) doesn’t often own the NPA-NXX block either. Around where I am, the blocks are generally owned by IrisTel, who is a SIP provider in their own right. An old client of mine that had a data residency/privacy issue (their client required all of their data to be processed in Canada) ended up provisioning some numbers directly with IrisTel and doing that integration using FreeSWITCH.
I hate that the fraudsters make it so that we Can't Have Nice Things, but I also see why and if anything we need more ways to add costs (calibrated to be manageable to spend once, but costly if you get banned daily) for account creation in a lot of places.
SMS is woefully insecure for multi-factor authentication, when we have TOTP and other open standards that work with local-only password managers.
It's because it's super cheap and simple, though, and that's about it.
Now, there's more regulatory teeth to go after the shady providers allowing this traffic.
The provider information isn't encoded in the calls themselves, there's essentially a (number of) centralized databases that can be queried to get provider information, out of band.