How to change iPad setup from one phone association to another?

https://discussions.apple.com/docs/DOC-5931

The iPad's phone association is determined by the Apple ID being used for iCloud, Messages and Facetime.

i.e if you need to associate the iPad with a different iPhone and its number, then you need to wipe the iPad and log in to the Apple ID used on that particular phone on the iPad.

See here: Make and receive calls on your Mac, iPad, or iPod touch - Apple Support