Originally Posted by crisambo
If you have a T-Mobile US SIM on a T-mobile 8320 device, you should be able to use wi-fi for calling and messaging between US and Turkey.
Mr. AF, what crisambo says here is correct. It's called UMA (unlicensed mobile access). The wifi acts as if it were a cell site in the US and you can make calls, get email, text message, etc. as if you were in the US. All it costs is what the same call would cost in the US.
Make sure the cell network connection is off and UMA shows rather than EDGE or GSM. If UMA isn't what shows, it won't work. If you have problems with that, check the wfi subforum and the post again with problems.
Note that if you call a number in Turkey it will be a long distance call since the network will assume you're in the US.
However if you don't have a provider that offers UMA, such as AT&T, then you can't.