I had a similar situation. I ride a bicycle and can't afford to keep and operate a car. And I can't afford to pay for a cell phone but need one in case of emergency in areas that don't have payphones.
I purchased an old, working analog cell phone with a bad battery.My cell phone wouldn't operate with the wall transformer without a battery in it. Didn't want to buy a new, expensive battery so i insulated the contact points on the battery then rewired the connectors to an outside "batterypack" that could be plugged and unplugged and recharged. The "old" dummy battery sat in the holder with a nice fit. The wires ran outside of the phone and were attached to a cheap battery similar to the original battery. It worked, but the only number I could dial was "911" emergency...but that's what I wanted..and no monthly bill to worry about.
It was also suggested:
That would probably be a better idea. My hack could be a last resort if you can't get a new, cheap replacement.