Read this about the #librephone by the #fsf. It's a great effort but as someone who has been involved in at least 3 different phone efforts - it's a daunting task.
The biggest problem is the modem which is a black box and trying to reverse engineer that is going to be quite the effort but there is also convincing the cell phone tower operators to accept it as well.
Without creating your own network of sorts - you're only going to have limited success. This project also needs to work with govts
 
      
  
             
      
  
             
      
  
               
      
  
             
      
  
               
      
  
             
      
  
              