I didn't answer and they didn't leave a msg

They called and left a garbled message. When I called back I got the message "this number is no longer in service". Isn't that illegal. .....

Two calls from this number last night. I didn't answer and when I returned the call I received the same message "number is no longer in service"

This number calls multiple times a day to my elderly Mother's phone.  Usually a recorded message, sometimes a person will pick-up when Mom answers.  They continually threaten that she will be arrested if she does not pay them in regards to registering a car that she does not even own.  Perhaps this is a debt collector(?), but she has told them repeatedly they have the wrong number and to stop calling, but the request is ignored.  Escalating to AT&T's Annoyance Call Center.  Return calls to the number give the same result as reported by others here, "This number is no longer in service."