At least a decade of development probably more..
you omitted that in your OP I assumed as you described an alternative to an ipad you intended to use similar coms tech and infrastructure, my bad.With an amateur radio (CB) you buy the radio once and that's it. In some areas you need to pay a license fee but it's nowhere as expensive as a cell phone subscription.
The idea is to make an amateur radio that sends and receives text messages... might use less bandwith that voice! Might be possible to work only on solar power.. worldwide.
for a broadcast network model you may also want to check out the aloha network
https://en.wikipedia.org/wiki/ALOHAnet as well as lora for ideas,,
(fwiw i researched aloha before the www the old fashioned way)
- ahhhh nostalgia
perhaps a otg/usb RF plug in and supporting sw could be a short term target to exploit the use of old phones as i/o devices??
sorry but you cant shake me of the conviction that reusing an existing device even only partially is 1000% greener than manufacturing something completely brand new.
And while money may not be your prime motivator you are living in a world where it is a prime motivator for many and if you can exploit that to your and your target clienteles advantage, isnt that a good thing?
btw who pays a monthly phone sub??? use wifi hotspots and a 2g pay as u go sim card you just need a 2g phone to set up the sim card initially (here in the uk at least) ..