How to disassociate AppleID from an old iPhone? I have given my grandson my old iPhone, but all the messages sent to my current one are still being received by that phone as well, even though it's a different number.

How can I prevent my iPhone to do so?
Here is the correct answer for you. It is currently the highest-voted answer to a very similar question which was asked previously.

Go into Settings > Messages and halfway down choose Send and Receive. Tap your AppleID on the top of the page, then choose Sign Out.

If you really are giving away your phone there are several other places you ought to sign out from as well:


Settings > FaceTime
Settings > iCloud
Settings > iTunes and App Store


If you'd prefer to reset the phone to its factory defaults, wiping all your data, apps, music, etcetera, then use Settings > General > Reset > Erase All Content And Settings. Then the phone will be brand new like you just bought it from the shop.