Maybe this will help. I googled it, a lot of places referred to cell phones having it because calls will go to voicemail while you're using the cell. Here's a link from AT&T that has more.
Call Forwarding- AT&T Wireless Support? In another forum, it mentioned being able to turn it off in settings but since it wasn't our phone and it was a date from last year, wasn't sure how relevant it would be. I know I saw it a few times right after I initialized the phone but then it went away. I changed so many settings, I also set up the visual voice mail, so I don't know what made it disappear. Do you have visual voice mail set up? Calls going to voicemail are what came up in the search so maybe it was the visual voice mail.... just throwing things at the wall to see if anything sticks.