First off, let me say that I have absolutely no experience in using this method so I can't off guidance other than what is available by google.
With that said, it sound like you need a SIP proxy service. I've seen free ones as well as paid ones. I don't know how well they work, but you basically change the sip port on your ATA (or softphone) to point to the SIP Proxy provider on their special port.
Here is one of the examples found on google
http://freesps.googlepages.com/
From their FAQ...
Q: How this free outbound proxy service works to bypass the ISP's blocking on SIP 5060 port?
A: Your ISP blocks 5060 UDP port which is famous port for SIP protocol, and 95% of broadband phone services are using SIP protocol, so in general the broadband phone can't be used in your ISP, because all SIP request messages sending to the broadband phone servers are dropped by your ISP. But, mostly, your ISP didn't block all UDP ports, they will open UDP ports like 53 (for DNS), 135, or any ports > 10000 for you. With the help of SIP outbound proxy server, your SIP message will send to the SIP outbound proxy server through this no blocking UDP Port, the outbound proxy server will then forward the SIP messages back to your broadband service providers' system.