Hello & Welcome to Just Answer,
Sorry to hear of any issue you are having with your phones.
To help you with this issue, may I ask:
1) The Model Number of the phone system you are working with? That model should be printed on a sticker on the bottom or back of the main base unit and START with KX-TG.
2) How many TOTAL cordless handsets do you have with this system?
3) What is the AGE of this system?
4) Do ALL the other functions within this one handset still function properly?