There is nothing worse than your provider blocking outbound SIP traffic (UDP 5060). There is a great service out now that will allow you to bypass your providers hatred for SIP.
Last I checked ClearWire blocks SIP. I am assuming this should work for ClearWire users. If it works please leave a comment and let me know.
A complete list of proxies is available at http://freesps.googlepages.com/
Last I checked the following proxies and ports were avaialable: