On several occasions over the past few days my home phone has rung with a mobile number showing. On picking up, I can hear a conversation going on quietly between 2 people who seem to be able to hear me.
On hanging up and dialling 1471, I get the previous caller's number, not the mystery mobile that's just called.
Also, intermittently I get a high pitched alarm-like sound on picking up the phone on top of the usual dial tone. This continues during any phone call I make.