Hi Jan,

glad you are interested in picking this one up! :)
I'm the last packager of Jami, so I might be able to help.

After a quick glance, here is the situation it seems:

Jami used to depend on their own fork of pjproject (as packaged in
Guix).
However it seems that recent versions have dropped the fork to use upstream
instead.  _This needs to be confirmed._
If this is the case, switching the pjproject input to use upstream
should work when updating Jami.

The rest should be mostly straightforward stuff.  Don't hesitate to come
back to me if you need more help.

Cheers!

-- 
Pierre Neidhardt
https://ambrevar.xyz/