I haven't watched Trupone very closely, mostly because I'm not in their sweet spot as a customer and the value to me personally is low. But they get a lot of attention in other quarters. Here's a mention from StartupSquad noting their recent big funding win.

Truphone gets big money
Truphone, the London based startup that enables free mobile-mobile calls over WiFi, has closed one of the biggest Series A round in the recent history. Truphone today announced a funding round of $24.5m(£12.5 million). The funding round was led by Wellington Partners, and participated by Independent News & Media and Burda Digital Ventures.

It was only last week that I had written about upcoming release of Fring’s WiFi enabled application and few other startups competing the this space. Innovative efforts by companies like Truphone, Fring, Gizmo, and Nokia  are set to disrupt traditional wireless business model. Truphone’s big funding round shows the reconignition this fact and that mobile over WiFi is a viable option especially for areas with poor wireless coverage while also enabling
free mobile-mobile international calls.

More complete writeup on TMCnet

Truphone Nabs Largest European Series A Funding of 2006
Mobile Internet telephony start-up Truphone has announded that it has received a funding round of $24.5 million, making it the European technology sector’s largest Series A venture funding of 2006. The funding was led by Wellington Partners, a leading pan-European VC, with Independent News & Media (a global media group) and Burda Digital Ventures (a global media group) joining existing investors Eden Venture and various angel investors.