Not sure about setting up with Skype but wouldn't it be easier and cheaper to use Groove IP.
If you get the paid version you can make calls over T-Mobiles data network.
It uses your Google Voice number to place and receive calls so you only have two numbers to deal with.
I got one off ebay for $3. They're not bad. I'm just using the back since I couldn't get the front to line up correctly and it really bothered me.
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The tmobile stores I went to told me that they couldn't sell just sim cards. They said the sim cards couldn't leave the store without a plan attached. Not sure if they were lying but I went to several different stores.