Who called from 2515456623

"Unknown Caller from Mobile, AL".  They leave no message.  Upon immediate call back, I got the "this number is no longer in service" recording, which usually means someone trying to conceal who they are by changing the outgoing identifier on their phone.

One of about a dozen unknown numbers from all over the United States calling me 6-12 times a day. If I answer, no one/nothing is on the other line.