How do I change the phone number ID on iMessage on iPad?
The only way to register a phone number as a Receiving/Sending id on iMessage, is via an iPhone. 

Meaning: you can only register an iMessage phone number as an id if you have access to that phone number and you activate your iMessage account with it on an iPhone. 

If you want your iPad to receive your iMessages (sent to your phone number) you have to:


Activate iMessage on your iPhone with your phone number and Apple ID (your carrier might charge you for this process);
[On your iPad] Go to Settings -> Messages -> Enable iMessage.
Then, on Send &amp; Receive make sure the Apple ID is the same as in your iPhone and your Phone Number should be listed.


If you have already activated iMessage on your iPhone, on your iPad under Send &amp; Receive you just have to log in with the same Apple ID as in your iPhone. 

I also advise you to set the same id for Start New Conversations From on all your devices.