I recently purchaed a Nokia N90 phone with the hopes of accomplishing what you suggested. Unfortunately, the N90 usese the Symbian OS, which is currently unsuppoted by Skype and Yahoo. I have a friend that uses some fancy Treo product and he has had better results.

As for 17951, it works just fine, but the numbers aren't any more attractive than pre paid.

Peter N-H, a passport and cash are the only prerequisites. I brought along a residence permit, just in case, but they never looked at it.

The main benfit of post paid, is that you save tons on your in-country calls.