If you are reading this, you might want to add a new feature or fix a bug, so reading these few lines might be of help.
If you want to add new features that would be used at high level, e.g. in a custom sketch, my suggestion is to start by first implementing wrappers for the required AT commands. Have a look at the header files in the folder src/commands for some examples. We follow the structure of SIMCOM's AT command manual and roughly each chapter corresponds to a header file. Add any commands you need in the existing header files, or make a new one. Commands functions should return an int8_t code signalling if the operation has been successful or not. If an ouput is expected from an AT command, it is best to pass a pointer argument which will be used to store the result.
When the low-level AT commands have been implemented, high-level functionality can be added either to the A76XX modem class (if conceptually the new functionality is a property of the modem, e.g. `GPRSConnect), or in dedicated client classes.