My wife just got an iPhone 8 Plus today. No issues to activate. Maybe you can restore the iPhone from fresh and start all over again?
I'm havingthe same problem as u. Until now there is no solution to this. I have contacted Apple support and they have been tremendously helpful to solve this issue with me.
However,I have tried to reach celcom and even go to blue cube to lodge a report. it has been 3 weeks since I encountered this issue and Celcom do not being very helpful.
I have tried to reset all the settings, Apple even reset my Apple ID but still the code couldn't be properly sent.
I keep getting "????y???REG-RESP?V=3;r=1219516422;N=................." and since I have lodge a report to celcom I stopped getting message but still I couldn't get the code and activate my iMessage and Facetime.
I am experiencing the same with you. By any chance, does this has anything to do with our new plans or new phones? if this has to do with the plan I recently changed, I want to change the telco company and do not want to pay any single penalty as it arises from Celcom itself and they can never resolve this issue.
Seems that all of us who are having this problem cannot seem to find a solution. Even Celcom's so called support does not want to get their hands on this problem.