It does sound as though at least one connection has been wired up wrong as mentioned ...almost classic case. You need to have the wiring connection codes checked on your wall plates..all of them as if a mistake was made on one,likely it was made on others. What's happening is you are hooked up on the phone side of the frequency and when a call comes in ,it drops you off..just like the old Dial-up days. You could do it yourself by checking what colour wires go to which connection behind the wall plate against another (i.e. a friend or relly). You will likely find that on the connection you are using,or the one before it on the circuit, is hooked up to the wrong pin.Whoever did the installation is at fault.
