Phone Call Problem - Other Party Can't Hear Me (Contact-specific)

I am running iOS 9.1 on an iPhone 6+ and I am running into a very odd problem.

When I am on a phone call with my mom (only with her contact) she can't hear me, but I can hear her. Every single other phone is fine, in both her case and my case. She is an iPhone 5S user (unsure of OS).

I have tried turning my phone on and off, and so has she. I've tried switching to airplane mode, then turning it off. I've tried plugging in headphones, unplugging them, I've tried speaker phone, everything I have been able to think of so far. And so has she.

For some odd reason, she has to call me on someone else's phone. Everything I read said I need a new charging port or something, that the mic and the charging port use the same cables. But it's perfectly fine with other contacts.

I am wondering if it is at all possible for this to be a contact-specific problem, I've never heard of such a thing before but it seems to be the case.