Sometimes, perhaps when you are in a contractual dispute or a family breakdown situation, you may want to tape your own phones calls with other people so as to have a record of what was said. You may want to make those recordings without letting the other person know you are taping them. Is that legal?
Now, just because something is legal doesn’t mean it is very nice and judges are often quite negative about allowing taped phone calls to be used in court, especially in the family law context. Nevertheless, such taping is lawful.