XMPP?

[trymeouteh]

Maybe consider using XMPP over Matrix? Here are some reasons to consider XMPP over Matrix

• Lightweight compared to Matrix
• Been around for about 20 years
• Supports encryption (OMEMO encryption)
• Supports messaging and audio calls
• Many XMPP clients (Conversations, Dino, Gajim)
• Free
• Open Source
• JMP.chat uses XMPP

Matrix seems to be a bridge to connect different messaging protocols into one while XMPP is a federated messaging and audio/video call protocol.

[swedneck]

This is explicitly a matrix bridge, it's not an independent project that happens to use matrix.

[dngray]

For XMPP you might want to see something like https://f-droid.org/en/packages/org.projectmaxs.module.smswrite/