Tom Browder (12021-12-04):
Even if Telegram has a Libre implementation, it still relies on
centralized servers operated by a private entity hoping to make profit.
For this kind of use case, I strongly urge to rather use something that
enables you to run your own servers.
The obvious answer for this is XMPP, aka Jabber. Installing a server on
Debian is very easy with the prosody package.