SIM900 (Cellular module) VS SIM900-DS

Does anyone know if there are hardware that separates these two modules, or if it’s all firmware?

It would have been nice to be able to used the PCM audio in/out ports on the SIM900-DS.

Schematics shows that these two are pin compatible on all “standard” stuff. V+, GND, antenna, Serial comm etc are all on the same place. While the DS version have PCM in/out clock and sync, the basic SIM900 have GPIO ports on those pins.

If we could use the PCM audio stream from the modem directly, it would discard the needs for an external DAC/ADC for audio in/out. Would save us hardware and resources on the G400 - and there would be less conversion of sound, witch is good for quality.