I just solved this same problem; my iPhone incoming calls were ringing on both my and my wife's iPhones. We were both sharing my Apple ID. Instead now with the new iOS8 you need to have separate Apple ID's.
-Need to create a separate Apple ID.
-Under Settings/iTunes & App Store, click on the Apple ID at the top, then sign out.
-Sign in using the new Apple ID
-Under Settings/General, go to bottom and click on Reset, then click on Reset Network Settings
-The phone will go through a reboot, but everything else on the phone will remain intact.
You can test by calling the number and see that it only rings on the appropriate phone.
If you're still having problems, try calling technical support from your cellular provider.