Comment by phh

Yes, TCP/ip and SIP, no magic packet, but: 1. You need to do challenges with the SIM card for authentication 2. On most carriers, "ims" is on a special IPv6 data connection that can't be used by apps.

The specifications are public, but yeah it requires a lot of work to get into.

That being said, I think neither of those limitations apply to Google's "RCS" (which bypasses carriers, so they can't get their own IPv6 connection and I don't think they can use SIM challenges for auth), so I think it should be possible in a 3rd party app.