I can't receive messages on my iPhone 4?? It will let me send them just not receive, and this happens on text and iMessage. Help anyone?!

I have a iPhone 4 on contract and every now and then it will stop receiving messages when I am in the middle of a conversation. I don't use iMessage as I sometimes go over the limit of my Internet data. So I stick with SMS. And even when I do try with iMessage I still can't receive messages. I don't know if this is a on fault on the phone but it's a really bad downfall if its common on most iPhones. Don't get me wrong but I love this phone I think it's great but it's annoying when I can't receive messages. And at this moment

In time I still can't receive them. Been happening for 12 hours now and it's never been this long usually, advice anyone?