Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of the issue you are having with your phones.
To help you with this issue, may I ask:1) The Model Number of the MAIN BASE UNIT of the phone system you are working with?
(the model provided is for a cordless HANDSET only).
2) What phone service provider (phone company) you have for this phone line?
I believe the "KXTGCA21" is the model of a cordless HANDSET and charger.
If you can, please find the MAIN BASE UNIT of the system, and look for the model printed on the bottom of that unit.
I have attached an image of an example system, showing the different components.