I don't have a technical explanation, but I had a few weirdly misconnected calls and even a call that changed to another person in the middle of a call in the analog cell phone days.
Way easier to happen on the old analog phone systems, when an unreliable relay would not react, or randomly flip. With digital networks and all their checksums and error detection, it's not become impossible, but at least you'd much much more likely just get a dropped detection or other "unreliability" fault than being connected to someone else.
Back when I grew up there would sometimes be some steange noises or eerie silence or something when we was on the phone. We used to joke that "they are just listening in", but I guess no one believed that except possibly my mum. (There wasn't much interesting going on.)