Truphone is a low cost voip provider for mobile phone handsets (Think vonage but only on your mobile). I haven't used it as it's handset specific application but I've met some of the founders.
Anyway so it turns out T-Mobile have decided to block calls to these numbers http://thomashowe.blogspot.com/2007/06/truphone-gets-blocked.html
It's kind of like the recent 712 area code case against AT&T http://saunderslog.com/2006/10/11/whats-with-the-712-area-code/
Will be interesting to see what happens here and hwo this is resolved (federal, state, or free-market) eg the recent Maine net neutrality law http://www.lawbean.com/2007/06/16/maine-becomes-the-first-state-to-pass-internet-neutrality-legislation/
P.S. Found a better article with more information http://gigaom.com/2007/06/15/tmobile-truphone/#more-9592