The 8KHz samples were encoded with relatively low encoding complexity PCM (G.711) at 8KHz. That gets to a 64kbps data channel rate. This was the standard for "t

The 8KHz samples were encoded with relatively low encoding complexity PCM (G.711) at 8KHz. That gets to a 64kbps data channel rate. This was the standard for "toll quality" audio. Not 8kbps. The 8kbps rates on cellular are the more complicated (relative to G.711) AMR-NB encoding. AMR supports voice rates from about 5-12kbps with a typical 8kbps rate. There's a lot more pre and post processing of the input signal and more involved encoding. There's a bit more voice information dropped by the encoder. Part of the quality problem even today with VoLTE is different carriers support different profiles and calls between carriers will often drop down to the lowest common codec which is usually AMR-NB. There's higher bitrate and better codecs available in the standard but they're implemented differently by different carriers for shitty cellular carrier reasons.