"Could not sign in to iMessage" Could not sign in to iMessage

An error occurred during activation. Try again.





Also on the iPhone we get:


  iMessage Activation
  
  An error occurred during activation. Try again.


With also:


  Activation unsuccessful. Turn on iMessage to try again.


These show from Settings -> Messages -> Turn on iMessage



We have an iPhone in the office and we use it through the iMac by using FaceTime on the iMac and Messages.app on the iMac.

This worked well for a week. Then FaceTime stopped working. Now iMessage stopped working. Also, on the iPhone, messages stopped working, we see this on Settings -> Messages -> Enable. It produces the same error there when trying to enable.

We have tried these remedies:


Restart devices
Log out / log in Apple account, iCloud, Messages
Create new user account on iMac


None of these have resolved the issue. What should we try next?
I also suffered from the same and in the end contacted apple support who made me generate a support pin after logging into Apple ID and then did something on their end to resolve the issue. they said I sent messages to people not in my contacts. 

but i still suggest checking if network provider blocks accessing iMessages.