At one time, GSM carriers (AT&T/T-Mobile) used EFR and AMR-Full Rate codecs, which offered GREAT quality audio.
AT&T killed it off early on with HR and AMR-Half Rate (Europe pretty much wouldn't touch Half Rate codecs).
T-Mobile (in many areas) kept AMR-FullRate. Older GSM phones could be programmed to 'force' AMR-Full Rate (old Nokias/ Motorola RAZR/Vxxx series), and it worked VERY well. No garbles/drops, and audio was VERY good.