For example, there's Challenger in which offers the opportunity to have a local mobile phone number which works all over the world. So you don't pay roaming charges and the caller doesn't pay overseas rates.
Naturally, this service only works on the usual set of Wi-fi enabled Nokia handsets. One useful facility is Challenger forwarding. That means that calls to your VoIP phone number are forwarded to your regular phone number.
That's useful when you're not online. However, it would be unnecessary if these guys could actually get Presence to work.
Incidentally, if you join Challenger you can not only make free calls to others using the Challenger service, you can make free calls to any other SIP service. Like Truphone, for example.
The INQ got quite excited when it found that Challenger's One4000 VoIP service worked on three Blackberry models - 8700, 8707 and Pearl.
Then we realised that you need a 8820 to get a Blackberry that does both cellular and Wi-fi. Never mind. ยต
L'INQ
Challenger Mobile