I'm a bit concerned about the enthusiasm for Telegram. The clients may
be FOSS but the server software isn't. In addition to this the use of
a central server is troubling with respect to metadata collection.
Has anyone considered Tox? It's a FOSS replacement for Skype and
doesn't rely on a central server. It's still in a pre-alpha state but
has several working clients. I've tested qTox and it works nicely
with sound and video through a firewall. There was a Gtk client (Venom)
but it's deprecated.
A large project like Fedora being interested in Tox could make all the
difference and increase traction to help move it towards a stable and
feature complete release. I've noticed several people mentioning OTR.
Tox has encryption built in.
I could go on about it but the information is on their website and the
code is available for perusal.