There are already some solutions available, but they do not use blockchain technology. What you seem to be describing is what is called a "Mesh Network", some apps already do something similar but of cause they are all limited by the number of users in the vicinity and works over WiFi or Bluetooth. All have to run the same Mesh app.
But I agree completely, that we should aim for decentralized future also when it comes to our phones. I think the biggest problem right now, is that there are no good radio frequencies to make it work perfectly because they are locked down and illegal to use for people/corp's that don't pay for the right to use them.