Default Unable to text one number

Interesting issue that happened Friday afternoon. I can receive text messages from my girlfriend (verizon) however I cannot text her (AT&T) MMS and group texting works. She has an Iphone, I had her block my number and unblock it, deleted the contact on both phones are tried. Power cycled both devices. AT&T is "Investigating" the issue however I don't think they will do anything. I figured if anyone had any idea's it would be you guys/gals.

Any other advice?