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...