Alex makes a valid point, success depends completely on the method they are using to block SIP traffic.
If they have DPI, then you can try using any number of VPN services to tunnel your SIP traffic (there are literally hundreds of them, though personally I find VyprVPN to be quite good).
If your equipment is hardware then you'll need a router/gateway which supports tunnelling traffic (ipsec, openvpn, l2tp, pptp etc), or use some routing trickery by changing the default gateway on the phone to use your computer then set up your computer with ICS (internet connection sharing), assuming you are using Windows, or DNAT if you are using *nix via iptables. Some hardware phones may also allow you to connect via a VPN.
Hope this helps
Cal