The easiest solution is to put the phone into airplane mode. This will turn off all radios in the phone. Once you do that, you can re-enable wi-fi and only use that.
Before you go, you might also want to grab an app called GrooVe IP. It uses Google Voice to make free VoIP calls to any U..S. or Canadian number. If you don't already have a Google Voice number, get one set up, as you'll need it. The app's instructions will tell you how to properly configure your Google Voice account to work with it. GrooVe IP is $3.99 in the Market. I've seen it cheaper on Amazon, but app updates tend to lag behind what the Market offers.
Hope this helps.