This library is derived from the RIL layer in the openCPU framework, which has implemented a similar application on STM32 processors.
For clone repository with all submodules you have two choices:
git clone --recursive https://github.com/hamsadev/ril.git # Version 1.65+
git clone --recursive -j8 https://github.com/hamsadev/ril.git # Version 1.9+
git clone --recurse-submodules -j8 https://github.com/hamsadev/ril.git # Version 2.13+
git submodule update --init --recursive
git pull --recurse-submodules