Imessage Help

Hi Guys,
When I try running Imessage it gives me a customer code and asks me to ring apple. I've been Following this Guide and it says that if I get a customer code then I should ring apple. However it also says that if my Serial Number returns the message 'We're sorry, but this serial number is not valid. Please check your information and try again' on checkcoverage.apple.com then I should get a new serial number. I just wanted to double check this as some people on the same thread are saying that its ok if that message comes up but the main thread says that I need this message, 'We're sorry, but this serial number is not valid. Please check your information and try again', to come up before I contact apple.
Which is correct?
I'm trying to get a working serial for an IMac17,1. I'm currently got a serial number that turns up invalid when checking on the checkcoverage.