Why does my upstairs phone crackle? ?

I have an old home with three phone lines. Basement line..works great. Main floor(cordless)works great. Upstairs phone very crackly and the line goes in and out. When you have the main floor phone and upstairs phone in use at the same time, the crackling of the upstairs phone goes away. Any ideas as to how correct this problem? All the phones have been checked and are in perfect working order.