Phone number issue

When I plug my iPhone 4S into iTunes my phone number is displayed as NA. Could this be causing my iMessage not to activate, and also to send iMessages from my email instead of my phone number?
Also, sometimes I'll check my email and there will be text messages that were sent there. Any ideas? I switched carriers about a year ago and after I was getting text on both iPhones? could that also be a problem. I've been trying to fix this for quite some time and it's getting frustrating. Thanks!