My iPhone won't send texts too one person

So my friend and I both have iphone 4s's, and we used to be able to text via both imessage and regular text message. But now whenever i try to text her using either, it just gives me the "Not Delivered" error sign. She can still text me though! We do have different carriers, I'm on Bell and she's on Virgin so I dont know if that makes a difference. Bell has even sent me texts saying "Your messages to ******* could not be delivered. Please check the number and try again." The number is definitely right. I can text everybody else, Both iphones and androids, so I'm not sure if this is apples fault or the carriers fault, but i really want this fixed!