According to a leaked document which is posted in the TmoNews T-Mobile will be adding Wi-Fi Calling services to its selected Android Devices. This will be similar to Skype on Verizon and will allow T-Mobile customers to have unlimited calling when they are connected with to a Wi-Fi hotspot.
As per the tutorial, T-Mobile customers will be able to send and receive calls and messages over their local Wi-Fi network. The service is completely optional and customers will not have to make any changes to their existing calling plans. As of now there are no list of compatible devices has been released, but Wi-Fi calling is expected to only appear on newer T-Mobile phones like the HTC G2, LG Optimus One, Motorola Defy, Samsung Vibrant, and I suspect the Galaxy Tab.