So when I try to text my girlfriend, it has an red X and says Unidentified Subscriber, She can call me and I can call her, and I can text other people. And she can receive texts from other people as well

I have whiped my phone, restarted, Taken the battery out and everything.

Could someone please help me (Us)

*And I have a 8700*

Wirelessly posted (White and Nerdy)

It sounds like there is an issue with the text messaging feature. Call your carrier and see if it is on their side. I have seen this before

It may also be a network issue with your carrier, I've heard of issues happening before if it's inter-carrier as well. As dankarlinski said, just call your provider.