Your 9700 has (rather, may have) the capability to connect to a wi-fi network. Find the manage connections icon on your applications screen and click on it, and go from there. Also read about wif connections in the Help app on the blackberry. In short, you enable wifi, then click setup wifi, and follow the instructions from there.
Some blackberrys and carrier networks support UMA (google it). This provides the capability to make voice calls over the wifi network. This capability is dependent on both the blackberry and the carrier. I believe (from quick look via google) that Rogers has it. Not all carrier-branded 9700's have it.
So, you setup a wifi connection, and when you are connected you should see UMA in place of GPRS/EDGE/3G, and voice calls will go via wifi instead of the mobile network.