Me and my friend just started working on OP.
We made it because everything out there that gives agents a phone number are just abstractions of Twilio, and only give out VoIP numbers. But these types of numbers don't really let agents be useful on the web, since VoIP numbers get blocked for most use cases like 2FA codes, form inputs, and callbacks.
That's why we think an API that let's an agent use a real carrier-backed number would be helpful. We really want feedback tho, so feel free to try the free tier on op.inc or op.inc/skills.md