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

If your phone supports e-SIM I would recommend something like Airalo. Source: 12 months ago

Alternatively look for a data esim like esimdb.com or airalo.com for regional esim you can install prior to departure. Can check the big Euro carriers for regional deals like Orange or 3, etc. Source: 12 months ago

I was just the Marangu route. There was wifi in the huts, but I bought e-sim for data. It was available almost everywhere. I used airalo.com. But find even more similar companies. Source: almost 1 year ago

Esimdb.com or airalo.com is your solution. Source: about 1 year ago

Suggest getting esimdb.com or airalo.com esims prior to leaving the US and use Fi data sparingly hoping not to get suspended. Source: about 1 year ago