Inbound XEP-0353 (Jingle Message Initiation)

Would be very useful to have XEP-0353 support for inbound Jingle calls in Gajim.

My company, JMP.chat, is willing to sponsor this work if that would be of interest.

I know other things are needed for compatibility with certain clients, but XEP-0353 alone would be very useful to us and would be a good step in that direction also.