Yes, there are many APIs available to look up the carrier that services a phone number. You can sort these carriers into categories (landline, mobile, VOIP...) and many services won't accept the number for SMS OTP use if the carrier isn't a "real" mobile carrier, in a somewhat hamfisted effort to prevent fraud.
As a counterexample I long ago ported my landline phone number to a SIP provider that supports SMS and due to the phone number being baked into various accounts the family has, I know it works for verification at least for those services (one of which is a bank).
You figured out the right way to do it: porting an existing number. That is a good workaround for the easy way to do “service provider lookups” via an NPA NXX database like https://localcallingguide.com/lca_prefix.php. I suspect that if you go there and enter your area code and NXX you’ll see it listed as your original landline provider and not your SIP provider. When you go to provision a phone number from Twilio’s own pool, you’ll often see that all of the numbers come from a small number of NPA-NXX-xxxx blocks and those blocks are the ones that many 2FA and user auth services reject.
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 ported my longtime cell number into GV a while back and have also noticed that it kept working everyplace where I ended up leaving it. I suspect they only run these checks upon the addition of the number, and not ever again.
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.
