Company I worked for, non-aviation, took on in the 1980s a lease from BA at Heathrow House, on the A4 next to where the DoubleTree is now. Probably built 1970s - still there. Had all five floors, BA had completely moved out.
Phoned a colleague up one day, all normal and we were having a technical discussion. Suddenly there breaks in a stroppy woman "Hello there, this is British Airways Switchboard SUPERVISOR here at (some location). We regularly have your company coming in on OUR lines like this. Give me the phone number you are on, I will have it traced. Yes ... and yes ... Good day".
Presumably some phone line connection not completely severed when they moved out.