SF, Although creating an open GSM mobile modem is an interesting technical exercise it seems to make sense ONLY if there are secure mobile networks within which to operate. Although someday that might happen alas this not even close to being the case (there are several OS SDR projects for base station/network infrastructure creation). All standardized mobile networks were created with significant influence from intel agencies and even if OS SDR networks were deployed they could not be made secure if they integrated with the S7 backbone/interchange of the current networks. Even if your project manages to yield the result desired it will still be stuck in the "matrix".
A much simpler, more secure and generally useful mobile system might result from mobile devices with hardended kernals/OS' using only WiFi in conjunction with good wireless hygine to prevent data leakage, a variety of available technologies to enable secure end-to-end voice and data communications. Secure paging, to alert users who's devices are not currently connected, should even be possible by adding a RTL receiver dongle and using appropriate tradecraft with existing pager networks (yes, they still exist and are used heavily by professionals (including sex workers), government and even devices calling for repair). WW PS: Suggest you enable lower contribution steps for crowd funding. They are too high for many who might like to help but cannot afford or justify the current steps.
