Right...VoIP calls can be made with as little as 8Kb bandwidth, so 2G will suffice. But given that the OP may not have signed up for the "smartphone" internet plan, instead opting for the "regular phone" web2go plan, then it may be instant fail since the plan's designated APN seems to route all traffic via somewhere near Kansas. The smartphone plan seems to route traffic closer to the state/city where you actually are, at any given point.

Latency, at best, is already pushing acceptable limits @ 150ms, on 3G/smartphone, but backing down to lesser speeds, and "farther/more hops", let alone what Skype may have setup (test call voice has British accent...hmmm), you're gonna have some QoS issues.

I experienced similar results w/Skype as the OP, but faired slightly better w/my Vonage and callcentric accounts via the native SIP client.

Now wall penetration for T-Mobile USA's signal leaves much to be desired. That's a whole other issue.