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 Make & Model Number of the phone system you are working with? If this is a cordless system, that model should be printed on a sticker on the bottom or back of the main base unit of the system (the LARGER base that connects to a phone cord).
2) How many TOTAL cordless handsets do you have with this system, and if MORE than one, do ALL handsets have this same "check telephone line" message showing?
3) What phone service provider (phone company) you have for this phone line?
4) If you have ANY other devices (phones, fax, etc.) attached to this same phone line and if so, are they working properly (get dial tone, can call out, etc.)?