http://bugs.launchpad.net/ubuntu/+source/empathy/+bug/449984 reports:

This bug actually covers some observations and issues with functionality in trying to make use of Empathy SIP with local SIP servers.

First, empathy always binds locally to port 5060. This is actually rather inconvenient especially if needing to run multiple user agents, and hence most SIP clients have a means to specify alternate ports for the user agent (and also for RTP). Related to this, rather curiously, empathy does not seem to register itself using the port SIP is bound to (and the SIP protocol is connected to), but from another, ephemeral UDP port that is created to send a SIP message. To support NAT situations, many SIP servers prefer to reply directly to the "apparent" port and peer socket address/port# where the SIP udp message appears to originated from, and this seems to explicitly fail in the case of empathy SIP. I was going to get a trace to see if Empathy sets a rport=5060, which I think would make sense to do in the given situation. Once I have time to do a trace (wireshark), I will comment on this further.

David Sugar wrote on 2009-10-13: 	#1

After examining this with wireshark, what I find is that empathy sip never sends an "expiration" for SIP REGISTER, and hence also never refreshes registration prior to expiration.