While the issue may boil down to coverage, it happens when my phone has full service and/or is connected to wifi. I'm using apps, can pull up youtube and use other high bandwidth apps just fine.

I'll get a text "why aren't you picking up? I tried calling 4x." I have no missed calls. The missed calls never appear on my device. In some cases, I call right back and I get through. In other case, I try calling and my call, too, doesn't go through. I switch to whatsapp and get through on the first ring, and the voice call via whatsapp works fine.