I have a T-Mobile Samsung T959v phone with a T-mobile SIM card. I have activated the phone and everything was working fine. Today I traveled a bit south of home, and within 4 miles of my house, the phone displayed "not registered on network" if I tried to place a call. The lock screen displayed "searching". The phone was receiving signal, often 4 bars. I tried rebooting it several times, but that did not help. When I returned home, the phone required another full reboot before it would work again.
How can I fix this?