How can I change the phone number in the apple ID and delete existing phone number ?

Thanks for responses and help. After quite some time online with Apple support it was agreed, that the wrong phone number and the non-receipt of an "activation code" due to wrong phone number was a somewhat "unheard" problem and potentially a phone provider issue. Thus trying that route - and since FaceTime does work on the iPad I will just be quiet for the time being. A pity that the problem of not working iMessaging and FaceTime on the iPhone could not be resolved.

Thanks, Eric, a lot for your patience and help trying to resolve "my code issue". I will try a bit later and who knows: suddenly it works (particularly with the "trusted phone call option"). Thanks - and most likely to a final "thanx a lot"......

The big problem is, that I don't receive the code to activate the change, which - by the way - seems to only be possible by ADDING another phone number; strange but that's how it seems to be.

I have signed in and dis-activated both iMessage as well as Facetime before