I don’t know what reminded me of this funny little event of the early 1990’s but hey.
Our exchange had a callback function which rings your extension when the one you are dialling becomes free. I had an answering machine connected to my phone, one of those old ones with two cassettes in. Anyway, I rang my boss but his phone was engaged so I hit the callback button and hung up. Some time later (he must have been on a very long call!) I had to go out and then went to lunch directly afterwards. When I returned there was a rather confused sounding message from my boss on the answerphone. What had happened is that his call had eventually ended and when he put his phone down the exchange then rang my phone to indicate the callback was available. My answerphone picked the rings up assuming it was an incoming call which resulted in the exchange then ringing my bosses phone while the answerphone was giving out my “I’m away…” message. He picked up to hear the end of my outgoing message, and my answerphone then recorded his confused mumblings to his PA, something along the lines of “…I don’t know, it seems to be an answerphone…”!