I have a new Decor 1200 which keeps ringing out with a long tone whilst displaying an "incoming call". When I answer there is noone there. None of my other extensions ring. I can get a dialling tone and make calls, but I can't keep the phone plugged in because it continually makes this ringing tone every 30 seconds or so. There are two other extensions on the same line which seem to work OK and three SKY extensions connected to a TV which are very rarely used. Could these be causing a problem? And do the TV extensions affect the REN rating?