iMessage not working in iPhone 3Gs

My son's iPhone 3Gs was recently replaced due to a smashed screen. The replacement phone was set up from an up-to-date back up of the old phone, however, iMessage is no longer working. When I go to Messages in Settings, iMessage is greyed out and there is the text, "Activation unsuccessful. Turn on iMessage to try again." Yet, as iMessage is greyed out, I cannot turn it on!

Any suggestions gratefully received.

Thank you.