In this update I have a very prototypey functional phone client. It's good enough for a proof of concept. Here's some highlights of what i have done. 

I hacked up some USB cables to get the phone client wired up along with a hasty audio patch cord to pipe phone audio between the PI and the cell module. We now have the Pi connected to a modified-to-be-powered USB hub. This powers the cell module and the USB audio adapter.  A 5 volt buck convertor takes the ~16 volts from the 4s battery pack/ BMS to energize the USB hub and the Raspberry PI. 

Minor victory achieved.