I just purchased an Avail and intend to use it only for WiFi at home and work.(No data plan wanted/needed. I can do phone calls with GoogleVoice.) Do I need to put in the SIM and "activate" the phone even to just use WiFi? If I ever do want to pay for service, it will be with T-Mobile, so I don't want to activate the SIM through AT&T. Will I be able to use T-Mobile service using the SIM that came with my Avail?