Like Carlos said when the work is finished making the Librem5 a “Matrix Powered” phone you should have a built in option.
See When will the L5 be a Matrix-powered smartphone?
Alternatives include using a Matrix, Telegram and Signal (for calls made between other users of those services)
There are also XMPP solutions but they (like the others) don’t encypt once the call goes across the phone network (ie calling through your XMPP client to order pizza is not encrypted).
An example would be JMP chat (https://jmp.chat/)