ETSI TR 101 294 Digital cellular telecommunications system (Phase 2+) (GSM); Subjective tests on the interoperability of the Half Rate/Full Rate/Enhanced Full Rate (HR/FR/EFR) speech codecs, single, tandem and tandem free operation (GSM 06.85 version 6.0.0 Release 1997)

standard by European Telecommunications Standards Institute, 01/01/1999