My husband and I both have iPhones. We have a 3G and a 4G. When we text each other the texts appear in a green box, not blue, thus we are charged for our texts. Shouldn't the texts between iPhones be in blue boxes? Can anyone help? We think it may be the 3G, that it is an older model, but should that matter?
Both of your devices would need to be registered with iMessage, and you can check that by going to Settings > Messages. Then you will want to make sure that both devices are connected to a good network, whether it be cellular or wi-fi. If that doesn't resolve this for you, try the steps in this article http://support.apple.com/kb/TS2755.