Iphone 5 and 6 problem
My parents both have iPhones, my father has an iPhone 6 and my mother an iPhone 5. They are both in the same apple account but have different phone numbers registered to the sim cards in their phone. For some reason when one of the numbers is called the other phone often rings as well, this is not a FaceTime call but rather a phone call directly to the specific number, the same was happening with their text messages, but I fixed the issue. I have already looked at the number and each phone is only registered to their particular number, how can I stop both phones from ringing when only the one number is being called.
Thank you for your help in advanced.
iPhone 5, iOS 8.4