Well that's going to be really, really easy to track down and prosecute.
900MHz is allocated licensed spectrum (some mobile phones too). Not only will the NSA be able to find you pretty easily, but the FCC will also throw you in a Federal hole for unlicensed spectrum usage.
Not only the FCC watch either. Cell phone companies watch the spectrum like hawks because they don't want noise interfering with their business. They'd locate you and dob you in within minutes.
Once the FCC have softened you up with some line that unlicensed transmission puts lives at risk and they can have you for attempted murder, you'll be willing to talk to anyone.
If you're going to rebroadcast then use unlicensed spectrum. 2.4GHz is a great place to hide out because it is the wild west of the RF spectrum. Noise does not stand out.