Without knowing your requirements, it's hard to make any real recommendations. Mumble, XMPP Jingle (groups possible) and Matrix all are options with audio (and some video) call support. Source: about 2 years ago

Take encrypted file transfer for instance. let's say I'm in an OMEMO chat session with you and I click my client's "send file" button. How do I know whether or not my file is going to be encrypted? there's enough wiggle room at the intersection of all of the relevant XEPs (not to mention client-defined behavior) that I'm not sure I could actually tell you for sure without looking at the source code for both clients. Source: over 2 years ago

Unnecessary for a chat protocol. In addition, if you want that you could just move to XMPP instead which does have XEP-0166: Jingle SIP-like session initialization. You could also use Mumble instead. Source: about 3 years ago

GoogleFU says this looks to be what you are after or at least similar https://go-text.me/. Source: about 1 year ago

On your phone download TextMe and create an account for it. Your textme phone number has to be in the US state where you're shipito address is (well maybe not but again, for safety's sake). Unfortunately it's not free to get a US number,but it's on sale now, I think it's like $2. (I don't know if there's any other app like this that's free, but this worked for me.). Source: over 3 years ago