Here;s the simple walkthrough to get to your Network preferences:
Turn on your Treo 650.
Press the "home" key (looks like a little house on the left of the directional pad).
Navigate to "Prefs" in the main screen.
In "Preferences", the Network preferences are under "Communication".
Since you're using T-Mobile, you'll see "T-Mobile Internet", "T-Mobile VPN", and "T-Zones" if you click on "Service". Make sure your handheld isn't locked though (you'll see the padlock closed).
In the Main screen you will also see Bluetooth. Access this and turn off your bluetooth radio.
One more thing...make sure you're using the latest firmware from Palm. There are features and capabilities that are necessary for your phone.
Go here: http://www.palm.com/us/support/
Download and install the T-Mobile or Unlocked (which ever applies to your phone) firmware and follow the instructions completely.