Seems like we’re better at recording the room’s acoustics than N900 because of the stereo mics. The modem’s audio does sound a bit different between the devices. I wonder whether it’s the PulseAudio’s intelligibility enhancer’s output being slaughtered by the GSM network’s codec? 
When I get some time for that I’ll try to play with some knobs and check whether it can be improved further, but it seems good enough to me for now so I don’t think it’s the most urgent thing to do at the moment. Anyone’s free to play with it by themselves and report the results though 