Hello & Welcome to Just Answer,
I'm Greg and will do my best to assist you today.
Sorry to hear of any issue you are having with your phones.
To help you with this issue, may I ask:
1) The FULL Model Number of the phone system you are working with? Since this is a cordless system, that model should be printed on a sticker on the bottom or back of the MAIN base and START with KX-TG, but have no "A" in it (that is for the handsets only).
2) What phone service provider (phone company) you have for this phone line?
3) Does this message display on ALL handsets in this system?