I think you're using such plain debian system already every day: maemo5 (maemo IS a debian system). Just remove all "cruft" from the init scripts and instead start a plain xterm or even console. GPRS setup, call initiation and accepting, sms - all can get done via pnatd and/or a few python scripts. Notification of missed calls is in pnatd log as well (hint "RINGING")
All that applies to N900 with stock maemo, and for sure will apply to Neo900 as well, since we're going to be maemo-fremantle compatible. The more it applies to a plain vanilla debian you may want to use on Neo900.

cheers
jOERG