To use it legally on real networks, that independent stack would have to be validated by the FCC, which is a big obstacle for a DIY project.

I'm a former DIY Cellphone hacker, and builder of several commercial GSM telemetry systems... Even using an integrated module you'll still run into FCC certification issues. Then there's PTCRB certs, CTIA membership, and carrier certifications. Of course even if you have the money you'll realize that all of these things have been done to the integrated module (which you haven't modified) before it ever hit the market.

OpenCell - my 2008 entry into the DIY cell phone game: http://web.archive.org/web/20080709054225/http://www.widgetr...

Fortunately, someone stopped them from re-inventing wheels, axles, GSM stacks, etc.

GSM is the definition of phone. Why not just call it "DIY handset" if that is what you are really doing yourself?