Why can I not receive phone calls, FaceTimes, or iMessages from one person?

Over the past few months I have had many problems trying to contact one of my contacts. There was no problem contacting him before but starting back a few months ago we were not able to reach each other through FaceTime. I would hit FaceTime and it would ring like normal but the call would never go through to his phone and when he would try to FaceTime me the same would happen. Next we could not contact each other through voice calls. The same thing would happen like it did with FaceTime, it would ring normally and eventually take me to the voice mail because the call never went through. When I would call him it would never show on his phone that he had a missed call and vice versa. Now when I text him or he tries to text me the messages don't go through for hours or days but when they finally do I get all the messages at once. He is the only contact that I cannot reach from my phone, but he has friends that can't contact him either. We decided that it must be a problem with his phone so we took it to the Sprint store but they could do nothing. We both have the latest operating system (iOS 9.3.5) and have tried everything we possibly could to fix his phone but nothing has worked. Can someone please help us