My impression is that SIP providers are usually not regular telephony companies. So by looking at who the provider is, it is often possible to determine that it is a SIP number and not a regular number. Which in turn might lead to the phone number you are paying for not being usable for account activation. Because websites will think that you are a spammer.
It used to be the wild west but FCC is tightening up (maybe even going too far, we'll see). STIR/SHAKEN and KYC (know your customer) rules are making it more expensive for providers to allow any traffic over their networks. Shady providers would look the other way at spammers pumping traffic (providers getting paid); shady providers mix legitimate traffic in so upstream carriers can't just block them, etc.
Now, there's more regulatory teeth to go after the shady providers allowing this traffic.
No, at least not without spinning up your own MVNO, registering it, and getting the phone numbers you care about ported over to it.
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.
