It sounds impossible for landlines to cross over on to cell lines and vice versa.
Also, I thought it was the Unibomber’s sister in law who noticed the writing that was similar to her brother in law.
You are probably correct. It was more likely the multiplexing equipment that allows multiple conversations to go on a single line or frequency that caused the problem. If the multiplexer at one end is not properly synchronized with the multiplexer on the other end, "ghost talk" can occur.