Edit: I am referring to mobile network local/national calls.
In: 9
It probably depends on your phone and network.
It helps to think about the phone data like streaming a movie. Even if you’re not really using data for the call, the way even wired phones transmit their data is close enough to data streaming we can just pretend they’re the same.
So when quality is good, all of the equipment making a connection between you and the other person is working well and you are streaming “fast”.
But maybe something goes wrong along the way. Some equipment may be overloaded, or maybe there’s some bad wiring somewhere in the network. That causes “loss”. Some equipment that’s part of the chain notices the “loss”. It responds by doing something to use less data. That usually means adding “compression”, which for voice data usually means cutting out some of the audio frequencies so each second of audio takes less data to transmit.
In a really sophisticated system like Netflix or Youtube and most internet infrastructure, the equipment will periodically switch back to “fast” mode just to see what happens. It might even try finding a way to send the data through a different route to see if that does better.
But phone networks are a weird mix of new and very old hardware. So some of it may not be set up to switch back and forth, and it may not be able to change the route the data takes once it finds a route. But if you hang up and redial, it might be likely the data takes a different route through different equipment and the new route is “faster” so compression doesn’t have to happen.
The behavior is by design.
There are many cellular connectivity standards, the current one is a VoIP based standard called VoLTE, basically it treats the call as a fancy Skype call, the data travels in the cellular and carrier backbone as IP based data stream.
Older technologies built for 3G or 2G (if they still exist) handle call data as circuit-switched, data frame based data stream, the protocol is proprietary.
When you walk into an area without proper LTE coverage, LTE protocol is designed to fallback to 3G and basically start another call in 3G to continue, you experience the drop in quality because the 3G technology is old.
However when you walk back out, it will not do the reverse by setting up a call in LTE. Because there is no guarantee that the 3G cell tower knows about 4G since they are invented before 4G. Even if the tower supports it, the tower side will not try to persuade the phone to switch, because the tower does not know if the phone making the call actually supports LTE.
Calls may fail if they implement the upwards switch, to play it safe and guarantee call stability for the entire network and possibly millions of users staying on 3G/2G, the protocol is designed to never switch upwards in a call.
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