As an alternative, if a firewall or NAT allows outgoing TCP connections, the inside client can open up a TCP connection to an outside proxy. All outgoing and incoming calls would then be handled by that TCP connection. (The client would still have to use SOCKS or similar mechanism to convince the firewall to let RTP packets through.)

Take a look at the two dratfs at http://www.cs.columbia.edu/~hgs/sip/drafts_firewall.html for a more detaled discussion of getting SIP through firewalls and NATs.