Somehow when I was in high school, my ex girlfriend's cellphone number became linked to my cellphone number so that if someone called me while I was already in a call, the new call would be redirected to her. It caused some awkward situations. I have no idea how it happened. I had once set her number as a speed-dial on my phone and also set it as a Verizon "friends & family" number so I could call her for free, so I assume somehow there was a glitch with one of these features. She was a mutual friend with a lot of my friends so people just assumed they had called her by accident, but years later I finally realized something was up when a new friend who didn't know my ex got redirected to her. I googled a lot and found out there was some phone star code you could dial to set or unset a number to be used as a backup number, I finally understood that was the association that must have been set, and I was able to clear the association. I wondered a lot if I had accidentally used the star code in the past to set up the association but I can't imagine having done it by accident, and I didn't know about the feature before.

Consider that she did it?

Seems an odd coincidence for it to be fwd to her number of all people.

Agreed, Call forwarding on busy or no answer. https://en.wikipedia.org/wiki/Call_forwarding Looks easy to set up and could easily be missed on the phone bill.

Nah, she was confused by the situation and I've always been the sort to keep my phone locked and to myself. She never had my phone unlocked.