Welcome to Android Central! When that person calls, does it go straight to voicemail? Open your Contacts app, find that contact, and tap the menu button--is it set to be blocked?
I had this happen to a close friend's number on my Note 3. Finally found that my friend's number had somehow gotten on my auto call reject list. I did not put it there! Go to your settings - device - call - call rejection. I bet that number is there.