I have experienced something similiar with stock a ROM. Things like this seem to happen sometimes. I have a Canadian Samsung Galaxy S7 with stock firmware and software. Only other thing I note is the device was originally for a different carrier. I never bothered flashing a stock ROM from my carrier.
I used to be on call for work. I would have to move around. Generally I would have excellent coverage and service. Sometimes I would go where no service could reach. I would also check in every hour or so if I didn’t hear anything.
Ever once in a while, I would receive calls that would start going straight to voicemail every once in a while. I had a radio as well. I’d get a buzz on the radio as they couldn’t reach me. My phone wouldn’t receive calls. I would restart my device to solve the issue. It would happen around once a month of 30hrs a week of on call work.
As I recall I would also be away from the radio a bit and call in and find that they hadn’t been able to reach me. Once I got off the phone, I could look and all of a sudden I would have notifications. This happened in a city of about 700,000 people with looked like fine reception.