If this is a NiCad battery it's probably suffering from learned charge. Leave the phone on for a full day to drain off the charge, then fully recharge. (Note: if the phone automatically shuts off below a certain charge level, go to that level, remove the battery and short the contacts overnight. Be very careful. This can cause overheating and even an explosion if there's enough charge left.)

This operation should only be used with NiCads. It can actually damage other types of batteries.