SIP + many routers is bad news - I've run into it many times, and I don't even do this that often... explains the issues... there are multiple spots were a ALG is supposed to mangle SIP packets for NAT support: http://www.voip-info.org/wiki/view/Routers+SIP+ALG
SIP signalling can be over UDP (more common) or TCP, typically port 5060.