Maemo Leste is pretty close to Debian already. Patches are upstreamed to the various projects it depends on. Unfortunately in the case of Debian, my experience is that they are not really interested in contributions from outsiders. Here's an example of a patch I submitted, completely ignored:

https://bugs.debian.org/cgi-bin/bugr...cgi?bug=893814

As for GSM, Maemo Leste already has SMS and 2G/3G data working with ofono although I don't believe they have been integrated into the UI yet. Phone calls have been reported to work but are currently of poor quality. See here for more info:

https://github.com/maemo-leste/bugtracker/issues/77
https://github.com/maemo-leste/bugtracker/issues/76

You may want to check out unicsy_demo, which has recently been packaged and is available in Leste:

https://github.com/pavelmachek/unicsy_demo

If you really are serious about getting this working and you believe you can help, I second sulu's suggestion and recommend that work together with the Maemo Leste team. If pure Debian is what you are after, it will be easy enough get telephony running on Debian once it's running on Leste.