How to make FaceTime ring on all devices?

I want to be able to ring my own iPad which is signed in on my one and only apple ID from my iphone and also be able to ring my iPhone from my iPad, but also when someone rings my number I want all devices to sound so that I can pick up the FaceTime call with any device of my chose.

Well sign into the Apple ID across the devices & check the settings to ensure that it can be reached using the same credential, like either the phone number or the email address.