imessage activated, but doesn't work.

[Hermoon]

hi.

i'm using an iphone 4 on 7.1.2, newly restored and activated. imessage and facetime are both activated but only facetime works as it should.
when i try to send a imessage to my 4s or ipad (all 3 connected to home wifi) the receiver text turns blue and the headline changes from "New Message" to "New iMessage" and the text box has a grey coloured "iMessage". but when i start typing my message the receiver text turns green and when i press send i get
"XXXX is Unavailable
This person cannot be reached by iMessage at this time."

but when i use imessage to send texts between my 4s and ipad everything works fine, its only my iphone 4 that can't use imessage. when i text my iphone 4 from my 4s or ipad it gets delivered as an imessage but my iphone 4 never receives it.

 

[stract]

I am also having iMessage problems. Basically, those who have iPhones who I have always sent iMessages to I can't anymore. I get the red exclamation point and it says "not delivered" and then I have to send it as a text message. I reset my iMessage (turned it off, turned off phone, turned on phone, turned iMessage back on), and it activated and I have a check mark next to my phone number, but still no change in behavior.

ETA: I have an iPhone 5, and have updated iOS to 7.1.2. I don't know if this behavior was occurring prior to the upgrade.