Another option you could consider is installing Asterisk (in extras-testing, usual warnings apply), connecting the SIP client to a SIP account on your local asterisk server, and then using an IAX connection from Asterisk to connect to your PBX or an IAX provider. IAX uses only one port (UDP 4569) and passes through routers far more easily than SIP. Also a lot of places that block SIP don't block IAX If you're connecting to an ITSP or a private PBX it's clearly the better option.