It is a codec offering a wide range of data rates, which was developed by the European Telecommunications Standards institute (ETSI).
The AMR codec is standardized for GSM, and is also chosen by the Third Generation
Partnership Project (3GPP) as the mandatory codec for third generation systems.
The philosophy behind AMR is to lower the codec rate as the interference increases and thus enabling more error correction to be applied.
The AMR codec is also used to harmonize the codec standards amongst different cellular systems.