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 BOTH of the phone systems 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 and START with KX-TG.
2) How many TOTAL cordless handsets do you have with EACH of these systems?
3) What phone service provider (phone company) you have for this phone line?
4) Did you try the UPSTAIRS phone system downstairs, and if so, did it work there?