EDIT: For others with the same problem: Phong was right, my friend had actually inadvertently blocked me.
I have an iPhone 6s and my friend has maybe the 6 or 5, but definitely an iPhone as well.
We used to receive each other's calls and texts no problem. Starting about a week ago though, he stopped receiving any of my texts but I still get all of his.
Also, any time I call him my calls go directly to his voicemail without ringing (and we tried enough times to make sure that his phone was on and should've rung) but he can call me just fine and my phone will ring.
I'm not having this problem with anyone else as far as I can tell (my gf and other friends get my texts and I can call other numbers).
One thing is that when I text this friend, I never see "Delivered" under the text.