It was BGP

"According to an industry source who spoke on the condition of anonymity, the root of the outage appeared to be related to how cellular services hand off calls from one network to the next, a process known as peering. "

https://www.cnn.com/2024/02/22/tech/att-cell-service-outage/index.html

First time in 15y I had issues.

It depends. Every carrier has better or worse spectrum and/or tower coverage depending on where you are.