Check out doing 1.6Kbps speech: https://jmvalin.ca/demo/lpcnet_codec/
Codec2 is also worth looking into as Alicja mentioned.
It would be interesting to see how xHE-AAC will perform at ultra low bitrates, but unfortunately there are no free encoders out yet that have the low-bitrate coding tools built-in.

@publius: most modern encoders are already tuned to resample to the best sampling rate based on the bitrate.
The audio quality from normal telephone audio running at 8Khz mono, 64Kbps is pretty bad. I remember the old days trying to hear each in a conference call