Intermittent phone problems. Between 7.00 and 8.00 in the evening I stop being able to make outgoing calls?

Next morning everything is back to normal. Occasionally but not always, I am also unable to receive calls - the caller gets engaged tone. I get dial tone, hear the number dialling, then back to dial tone and after 20secs a message to redial. I have disconnected Sky and broadband. Still the same. I have removed the cover of the master socket and plugged the phone into the socket there. Everything is OK then! Yes, this points to internal wiring, but I can't see how this can just start in the evenings and be ok again next morning (with everything apart from the telephone disconnected). There is no alarm system etc connected to the line. BT say they have checked everything and it must be wiring in the house.