Why can't my Xperia Z3 text or call one specific number (iPhone)?

[AC Question]

I have a Sony Xperia Z3 on the T-Mobile network and as of two months ago now, I have been unable to call or text my girlfriend.
She has an iPhone 5 also on the T-Mobile network.
When the problem began, we had been texting for half of the day and she suddenly stopped receiving my texts.
Now the texts show as delivered on my end, but she never gets them and when I try calling it goes straight to voice-mail. She can receive texts and calls from everyone else, while I can also text and call anyone besides her. We went through both our phones and I'm most definitely not on her blocked list. Tried restarting and re-adding each others numbers but nothing happens. It's been two months and I'm about to lose my mind. Could anyone help me fix this or explain what's happening. I would truly appreciate your help,
Thanks!

 

[Rukbat]

Go with her to her carrier. Let them see the problem. And hope they have someone who will make the appropriate calls to tech support at a high enough level that they can follow the call and figure it out.

 

[BenL]

Have her check your entry in her address book and see if she accidentally told her iPhone to block your calls.