How can call-forwarding happen randomly without user intervention?

[AC Question]

I started getting missed calls, but no voice messages. After trying to troubleshoot, I spoke to service provider rep (AT&T). Unanswered calls were being forwarded to another number - this NOT being set by me and to a number unknown to me. My phone is always locked and not accessible to tampering. The rep said he has seen this before and it happens randomly. He would not give me the forwarding number except for the area code. How can this happen?

 

[B. Diddy]

Welcome to Android Central! Did the rep elaborate on why this seems to happen randomly? It sounds like some AT&T network glitch.