Internet Low Bitrate Codec (iLBC) is a royalty-free narrowband speech audio coding format and an open-source reference implementation codec, developed by Global IP Solutions (GIPS) formerly Global IP Sound (acquired by Google Inc in 2011).
The bitrate is fixed at 15.2 kbit/s for 20 ms frames, 13.33 kbit/s for 30 ms frames as is the frame size (304 bits per block for 20 ms frames, 400 bits per block for 30 ms frames). iLBC handles lost frames through graceful speech quality degradation with a sampling frequency of 8 kHz/16 bit (160 samples for 20 ms frames, 240 samples for 30 ms frames).
You can download the library as zip and call include Library -> zip library. Or you can git clone this project into the Arduino libraries folder e.g. with
cd ~/Documents/Arduino/libraries
git clone pschatzmann/arduino-ilbc.git
I recommend to use git because you can easily update to the latest version just by executing the git pull command in the project folder.