In my home I have a DSL connection but also I receive a WiFi signal from one of the Apartment Stores of the near Mall.
The WiFi network is open (no auth required) but I couldn't register to Voxalot because somehow port 5060 is filtered or blocked, althoug I could register to FWD (because it uses uoutbound proxy with port 5082).
Today I was searching "tunnel sip" on Google and found a way to tunnel SIP, in this site:
http://www.realtunnel.com/ there is a client that allow bypass the blocked 5060 port.
Hope it helps someone else.