9/11 it became difficult to get through to my sister after the second plane it. I think the networks were just overloaded. Both cell and landline I had trouble for several hours. We were talking on landlines after the first plane hit, and when the second hit I hung up with her. When I tried to call back 10 minutes later I couldn’t reach her.
I barely have reception for cell at my house, but if I go out to the street it is ok. I am at the end of the range for the nearest transmitters.
Driving through Arkansas last weekend there were many dead spots. No signal, not even roaming.
Areas that are very hilly or mountainous have a lot of dead spots.
When I lived in Boca Raton, FL there was a dead area, it was surprising considering the density of the population. Finally a site was put up (a friend of mine built it) and the area now has a signal.
I don’t think I have ever had trouble getting a signal in most major cities though. In rural areas there might be a short time without, when traveling across country for instance, but shortly a signal is picked back up.