iMessage not working despite wifi connection

Hi,

I own an iPhone 3GS and have been able to use iMessage (sporadically) since it was introduced. Unfortunately, it now does not work despite a full wifi/3G connection on both sides. iMessage has been activated, 'send as SMS' has been activated and the phone has been rebooted several times but iMessage still does not work. I have heard that attempting to facetime another iPhone can help, but I don't have facetime. I'm on iOS 6.1.3 and cannot upgrade to iOS 6.1.6 as my phone is too old. Any support would be appreciated.

Thanks,

Mitchell