Changing mobile number on ipad

Hi,

FIrstly i will apologise as this has probably been asked before but I can't find the answer anywhere!

I have recently purchased an iphone 6 and an iPad air. Each has their own SIM card and obviously their own telephone number. This is where my problems start!

My iPad is showing my iphone telephone number for both iMessage and FaceTime. All I'm trying to do is change the phone number on the iPad to the telephone number from the iPad sim.

I have attempted to change via settings on the iPad but I can't seem to change the iPhone number that is already listed and cannot add a new number. I thought I might be able to add more numbers to my Apple ID but this has also not been possible.

Surely there must be a way of doing this?

Thanks in advance for any help!

Andy