why can i not text/imessage one person?

I have an iPhone as done my friend. We both have Verzion.

For the passed 2+ weeks we can't text or iMessage one another. We've both reset our network settings, reset our phone settings, checked our blocked lists, signed out of our iClouds, etc. to NO avail. She can get my messages, however they'll only send as SMS messages, BUT if she responds, I never get the SMS.

Apaprently this is a common problem but I can't seem to find a way to fix it. Does anyone have any input?