A setup like this but in phone size would be ideal. The free software mobile world has really been a dumpster fire from the beginning.
That modem or a few others can be used with some of the small SBCs, which have a smartphone size or even a smaller size.
If you connect then to some battery pack with a USB charger, you have a mobile solution, though one more cumbersome than a real smartphone.
Yes, whoever can shrink this down to the size of something that would fit in your pocket would make a killing. Imagine not only being able to make phone calls, but use productivity software and even browse the internet.