I have the same problem. I upgraded my daughters and my phone (ATT network) to IOS 8 and now calls go to both phones. In my case it only happens when we are both home on the same wifi network.
I think it has something to do with the new entry that shows up under the wifi settings. It seems to create an automatic wifi hotspot even though both phones have the "personal hotspot" setting off. If I tell the phone to forget the created hotspot then it creates a new hotspot on the other phone (over and over). Also my iPad rings when my phone does and even if I answer the phone the iPad keeps ringing for about 2 rings after I am talking to the caller (very irritating). I have both phones on the same Apple ID so I though this might be the problem but I can't find anyway to change my daughters phone to her Apple ID (I used one so she could buy content with my credit card off my id).