IPhone 4 - Unable to turn on iMessage but Facetime still works?

The phone has been kept up to date with software. When I go to my settings to where you can turn on imessage, the little switch to turn it on is grayed out. Facetime works fine - whenever I try to find an answer on google it's "facetime and imessage not working." Please help! Really need to fix this. I'm not able to turn it on ever even under Wi-Fi.

Apple has a Support Community with many Answers already online, you need to either search for the 'already answered question' or simply 'ask a question', I'll provide the link…

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