How to fix 'iMessage Activation Failed' error on iPhone?

Hi Apple Support,

I’m having trouble activating iMessage on my iPhone. I keep getting the “iMessage Activation Failed” error i don’t know what’s causing this please fix 

[Re-Titled by Moderator]

Original Title: Hi Apple Support, I’m having trouble activating iMessage on my iPhone. I keep getting the “iMessage Activation Failed” error i don’t know what’s causing this please fix