On a recent visit to US I bought a pre-paid SIM for use during my visit.
On my iPad this number (which is now expired) is still shown in a number of places - in particular "You can be reached by FaceTime at".
How can I remove this number?
You can disassociate/remove the number from FaceTime/Messages by following the steps outlined in the Apple support document, Add or remove your phone number in Messages or FaceTime:
Remove your phone number
If you don't want calls or messages for your iPhone on your iPad, iPod touch, or Mac, you can remove your phone number. There are two ways to do this:
On your iPhone, go to Settings > Messages > Send & Receive. Select your Apple ID and tap Sign Out. Then go to Settings > FaceTime, select your Apple ID and tap Sign Out.
If the phone number that you want to remove belongs to an iPhone that you can't access, change your Apple ID password. This removes all phone numbers from your iPad, iPod touch, and Mac. Calls or messages for the removed numbers won't appear on those devices anymore.