Codec2 is an open source low bit rate speech codec designed for
communications quality speech at 2400 bit/s and below. Applications
include low bandwidth HF/VHF digital radio and VOIP trunking. Codec 2
operating at 2400 bit/s can send 26 phone calls using the bandwidth
required for one 64 kbit/s uncompressed phone call. It fills a gap in
open source, free-as-in-speech voice codecs beneath 5000 bit/s and is
released under the GNU Lesser General Public License (LGPL).