We haven't done systematic testing of Fargo with various SIP clients and servers, or with various XMPP clients.
Repurposing this bug for short-term targets:
XMPP clients: Gabble
XMPP server: ejabberd
SIP servers: OpenSER (simple SIP server for testing) and genuine SIP server
SIP peer: telepathy-sofiasip and a hardware SIP phone
(Others: SIP peer (gateway): alsuren has tested with voipuser.org. SIP servers: smcv has tested with voxalot and sipphone.)
Estimated time 15h including potential tweaking for compatibility.