iMessage not delivered

I just can not send an iMessage through my iPhone 3GS – baseband 06.15.00 – iOS 5, it gives me an error message "not delivered" while I can do it easily using my iPad 1 – and using the same Apple ID

I don’t know but I have sent before using the iphone but I am not able and sometimes I could send myself but not the others.

I really need a good fix. But not restoring my iPhone cause it really sucks.