This just happened to me and I am a little shook. Very weird. I was talking to my friend in NYC and randomly this voice came on the line saying “this call is being recorded”
He heard it as well. We both have iPhones and were just talking over the PSTN. Wtf
Not part way through a phone call. Only as part of the initial call setup or part of the on hold loop.
If it has only happened once then it might be your call was routed a weird way around the phone network.