Incoming calls ring on two phones??

I have an Iphone5S and my wife has an IPhone4. We also have an Ipad 4th gen w/wifi. I am the primary at&t account holder and her phone is secondary on the family plan. The Ipad does not have a phone function. When my wife receives a phone call at home and I am also home, after it rings a time or two on her phone, it also starts ringing on my phone which is a separate number and it also may ring thru on the Ipad which does not even have the phone function.. It will subsequently show as a missed call on my phone but any voicemails that are left for her only show up on her phone. Both phones automatically log on to our home wireless network which may play a roll but not sure. I am inclined to think that the wifi is involved somehow as the ringing on the Ipad without phone is probably only possible due to being on the home wifi network. Might also be when we are in close proximity to each other or both logged into any common wifi network but we have yet to confirm that by being away from the home when it happens. Curious if any one knows why this might be happening.