The main problem is that those old phones use networks that are discontinued or use signals that are seemingly unable to penetrate some common types of building

The main problem is that those old phones use networks that are discontinued or use signals that are seemingly unable to penetrate some common types of buildings. At least, I think that's what's happening. I've had a number of older phones recently, and they would tend to have terrible service in the middle of the city, and absolutely no service inside a lot of buildings, especially office buildings or basements - both places where most people had full service.