There are two places where I encounter it, going from the Mid-Cities to Fort Worth, I lose it just after the bridge over the Trinity as you are passing Richland Hills and entering Haltom City, that one lasts the longest but it does eventually reconnect, the second one is as you are passing Sylvania and approaching I35, that one is very quick. AT&T is aware of both issues, the one at the bridge/trinity is because there is no place that they can put a tower that will cover 121 due to the river and flood plain. This affects all carriers. The second one is more of a mystery to them but has never been fixed. I also get a deteriorating signal at Campbell and lose it shortly after that for about a minute. Same thing near Bachman Lake due to the airport.
There won't be anything anything that Apple and the carriers can do about the ones where they can't place a tower due to property/radar/airport. There is also a spot in Reston, Virginia where everything fails, including the car navigation but that is for a different reason and again nothing can be done.