Found here is a handy list of Telos Zephyr Transmission modes. Modes can be individually selected for the Transmit and Receive paths.
Stereo and Dual-Mono Modes using two ISDN "B" channels:
Layer III Joint-Stereo at 20kHz or 15kHz for maximum fidelity.*
Layer III Independent Stereo at 20kHz or 15kHz for lower delay and surround-sound transmission.*
Layer III Dual-Mono at 20kHz or 15kHz when each channel has unique audio.*
Layer II Joint Stereo at 20kHz for compatibility.*
G.722 Dual-Channel at 7kHz for compatibility and/or lowest delay.
Mono Modes using one ISDN "B" channel:
Layer III Dual/Mono at 20kHz or 15kHz for maximum fidelity.
Layer II Mono at 7.8kHz or 9.8kHz for compatibility (48 kHz sample rate).
Layer II Mono at 8.6kHz for compatibility (24 kHz sample rate).
G.722 at 7kHz for compatibility and/or lowest delay.
Mono Modes using two ISDN "B" Channels:
Layer II Mono-128 at 20kHz for reduced data compression and compatibility.*
Split-Channel Modes using two ISDN "B" channels:
Individual mono signals are sent via Layer III and received via G.722 from separate sites.*
ISDN Telephone Mode:
G.711 is used to call a standard POTS telephone for low-grade voice communications over a single "B" channel.
* These modes require a stereo capable Zephyr (such as Models 920X or 9302)