This is not the typical thread on problems with iMessage activation after a port. I'm aware of the issues with the time it takes for the provider Apple uses to update the SMS routing tables.
I've just ported my wife's number from Vodafone to 2Degrees. It's prepay for now and she has credit on the account. I have verified that the phone number in iPhone's settings is correct.
In my past experience with iMessage activation after you toggle iMessage off and back on again, an SMS is sent to an international number. If a reply comes back, you're good, if not, you'll just see the message "waiting for activation" and it will be necessary to give it a while longer and try again.
However in this case, as soon as I toggle iMessage on, a pop-up screen appears saying that an error occurred with activation and to please try again later.
Checking usage, I can find no evidence that the SMS was sent, so I'm wondering if the SMS is even going out.
She can send other texts OK so the message centre info is right.
Any thoughts from @SaltyNZ or anyone else at 2Degrees would be much appreciated. Customer Care could only recommend the usual steps, all of which I'd already tried before contacting them.