Follow the instructions to install rustup at https://www.rust-lang.org/learn/get-started on your raspberry pi.
Currently:
curl https://sh.rustup.rs -sSf | sh
Then run rustup
to install the latest version of the rust toolchain and start a new shell so that modifications to your environment have taken effect.
Copy the example .env file and provide values for your twilio account:
cp .env.example .env
Warning, the first compile takes hours on a pi
cargo run