47 CFR Part 22.925. This covers the cellular band of 824-894 MHz.
Does not call out the modulation or whether digital or analog.

That's the one, and there's the rub. Most (all?) of the carriers use
digital signals on 800MHz cells. Some (most?) can also use a newer
1.9GHz network. You have to know what kind of phone you have, and what
network it is using at a given time.

My Verizon phone displayes "1X" when it is in a 1.9GHz network, and is
therefore legal to use in the aircraft. It says "D" when in a standard
digital network and "A" in an analog network, neither of which is legal.

The 1.9GHz cells don't tend to broadcast or receive at extreme
angles---they are often unusable in the air above about 3000AGL. Also,
at 140kt, it's pretty easy to go from one network to another quickly, so
I normally only attempt to use it below 3000 and either in populated
areas or along interstate highways.

Of course none of this counts the numerous times I have left the damn
thing on and discovered it at 10k feet. I have yet to be pulled over by
the cell phone cops, or hear of anyone who has, so I wouldn't be too
concerned in any case.