since last updating my phone, all my phone calls are being received by two phones
I did the last update yesterday, now my phone calls coming into my phone are being received by my son's phone also.
iPhone 6, iOS 8.3
Go to Settings > FaceTime and turn "iPhone Cellular Calls" to off on both phones.
You may also want to consider/investigate using separate Apple IDs. It is generally a good idea for each person to use their own ID and is more flexible, especially given recent changes with Family Sharing https://www.apple.com/ios/whats-new/family-sharing/