Go to Settings>Phone Check that both phones have their own number. If the numbers are correct, then check FaceTime, and make sure the numbers are correct there. If so, then on both phones, go to Settings>General>Reset> Reset Network Settings. Tap that. Your phones will shut off, then start back up. You will need to enter your WiFi passcode back in afterwards.
I've had to turn off iPhone cellular calls on my iPad, too. I got a cellular phone call to my iPhone yesterday that I answered on my iPhone. Today, I noticed a missed call badge in FaceTime on my iPad for the same minute. Very annoying.