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.