Hi Sterling R,
thanks for the links. Unfortunately the information didn't help. I had seen this and other information already.
Here is what I have tried so far:
- turned off iMessage and FaceTime on the phone and restarted the phone
- logged out of iCloud on the phone and restarted the phone
- reset network settings
- reset privacy and location settings
- turned off iMessage and FaceTime, turned off phone, re-inserted SIM card
the issues remain.
When I turne on iMessage and FaceTime on the phone, only my phone number shows up under setting -> iMessage -> send&receive.
If push 'use your Apple ID for iMessage' I get an error message 'iMessage Activation An error occurred during activation. Try again'

In addition, I can't log into either iMessage or FaceTime on my MacBook Pro, iPad Air and iPad mini. The error message is the same as above.
I read in one of the posts that it might be an issue with my account on Apple server side.