As I understand, Jitsi supports SIP.
SIP is very universal and is the usual exchange protocol for VoIP bridges based on Freeswitch and Asterisk.
One possible endpoint for these is a GSM modem allowing bridging SIP <-> GSM.
Do any of the Jitsi clients support GSM voice calls and SMS reception and origination over SIP?
If not, what would it take to develop them (in technical terms) - especially for the Android Jitsi client?