+1 for this suggestion. I have a T-Mobile Blackberry 9700, and the UMA calling is its killer app.
With UMA, the phone contacts T-Mobile over WiFi whenever that's available. That means that you can get T-Mobile service on your campus in exactly the way you would if you were talking to a T-Mobile tower: voice, SMS, Blackberry Email, Data Services, etc.
It's also amazing for roaming. Are you overseas? Connect to a WiFi network and all of the sudden you're in the U.S. as far as the phone company is concerned. I travel very frequently, and this feature plus a Boingo Mobile plan and Google Voice allow essentially limitless talk for +$8/mo on top of the ordinary phone bill.
For T-Mobile UMA, you do NOT have to buy HotSpot Calling as an option. (I don't think it's offered anymore anyway.) If you don't have the HotSpot option for $x/mo, you can still use UMA, but all minutes come out of your regular voice time (note free evenings/weekends still apply). If you have the extra option, then all minutes on WiFi are free.