I think you mean IMAP. Protonmail supports SMTP (the mail-sending protocol; it needs to to interoperate with other mail servers) but not IMAP (the client side mail-reading protocol).
> do people not use multiple phone numbers in USA?
Not really, no. Dual SIMs are kind of a niche feature here, and most users I've heard of do it with a US SIM plus one for some other country they travel to frequently.
Curiosity: what are the use cases for two SIMs from the same country?
If (you're into microoptimizing things and) all you care about is whether they're mobile, you can leave out `Type=caller-name` to save a whole one cent per lookup :)
And if you're doing this before you try to SMS them, you can skip the lookup entirely, if it's a landline you'll just get an error back.
(I work at a company that uses Twilio heavily and it's full of this sort of microoptimization)
That is not as reassuring as people seem to think it is.