Unable to activate FaceTime or iMessages on my iPhone 6S plus running iOS 9.2.1

THanks Demo for the links... Those trouble shooting tips did not solve my problem though. I think I have to go for a full restore. Will let you know what happens . Thanks once again

I wasn't able to get it done so far. Tried everything you've mentioned. Called Apple care as well. They asked me to restore the phone... Which I did and still cannot activate FT nor iMessages. I have a feeling there is some issue with the activation server at Apple. I faced a similar issue while first activating the new phon. Tried everything and it was Apple care that told me the activation server was under maintenance. The matter got fixed that evening as they had promised. But this new problem of FT and iMessage is quite perplexing. Tried all possible solutions provide. Can anyone help?

My service provider is AirTel and I had enquirer with customer support regarding the same. They replied saying that everything was okay from their end. But I strongly suspect that too. The reason being... I used my son's SIM on my new 6S plus and it turns on FT and iMessage in a matter of few seconds. When I put my SIM in his phone... It doesn't activate and the problem starts again. So I'm inclined to believe it is a carrier issue now. But the guys at support keep pushing me back saying it's all good at their end. Got myself in a pickle... Anyway thanks Kilted Tim for your response. Will let you guys know how this turns out.