It's not the nitdroid team's fault you can't make phonecalls yet. The ability to make calls on the n900 is behind a proprietory application and no source code is available to make it work.

The only option is to wait until the ofono project reaches a mature state where making calls and texting(?) is reliable on the n900.

I believe members of the MeeGo project are working on ofono so you can either wait until it works or if you have some programming skill contribute to the ofono project yourselves.