Not receiving messages or phone calls from one person, but receive messages on my computer

Since December my sister tries to text, call or facetime nothing happens on my phone... I receive no notification of any kind or any type of message. She said the phone rings twice and goes directly to voicemail. I have an iphone 6 and ios 9.3.1 ... We've realized that its my phone with the issue and Verizon seems to think that it is an apple ID issue possibly since I link my messages to my computer, but its only happening with one person and my email isn't even linked on this phone as a receive and send.

My Imessages go through to her, but do not say they were delivered on my phone. I can call and facetime her no problem, but when she tries to do the same to me it does not go through. As soon as I open my computer I get all of the messages she has been trying to send me or any message she has sent in a groupchat we are both in. Her contact isn't on do not distrub or anything like that so I have no idea what could possibly be going on.

Its got to the point where my sister and I have to use groupme to get in touch with each other and its really inconvenient. Has anyone else had this problem or know how to fix it?

I figured the issue would fix itself or that it was a glitch, but it has not and I can't continue anymore like this. If this cannot be resolved I'd honestly consider getting a different kind of phone other than an iPhone.