In the first generation of the Windows Phone 7 development software, access to sockets is not available--that's what is needed for VOIP services to work and why Skype won't (really can't) hop onto the OS just yet. Microsoft has stated that sockets support is coming, so with one of the OTA updates (fingers crossed), this will be made available.
Until then, a hybrid-approach can be used. Such method is used by companies like iSkoot where instead of using the data connection to make the free call, it re-routes the call to a local call center which then makes the call for you over the internet. It's like Opera Mini vs Opera Mobile--in the former, the "hard work" is done off the phone remotely. However, iSkoot hasn't updated their Windows Mobile software in sometime and they haven't said anything about support for Windows Phone 7.
Step in Voxofon who operates in a similar manner. They seem to be supporting all the new OSs in town, including pushing big into Windows Phone 7. So kudos to them for taking that market seriously. Now unlike Skype-to-Skype calls over true VOIP, you are charged for making calls on Voxofon. This is done like Skype where you can buy credits and they are linked to your account. Luckily, rates are very cheap making it an affordable option for making inter-continental calls: (1.3 cents per minute).
The software looks quite nice--it's a separate dialer app that log ins to your account, displays your balance and tells you the rate per minute before you call. It's no true Skype solution, but at this point we'll take what we can get.
[tipped by Alexey G of Voxofon]