[jimktrains2]

In the us you're allowed to receive everything but the cellular phone spectrum without a license. You or the device needs to be licensed to transmit, however.

Note that this isn't the case everywhere. I believe in the uk it's illegal to listen to air traffic, but I would need to look up the specifics to give anymore detail.

[R0b0t1]

Receiving of certain bands is prohibited, from memory the ones used on cordless phones and something else. All devices sold, say frequency analyzers, have to have a bandgap for them to pass FCC validation. It is common to remove these bandgaps and ignore the law because everyone seems to accept they are really dumb and they wreck the sensitivity of the instrument.

[anilakar]

The bandstop filter requirement is mostly a stopgap measure from an analog era when cell phones were basically trunked FM radio systems. That limitation serves no purpose nowadays.

[R0b0t1]

It's not cells or even maily cells, it's a weird cutout for in home wireless phones and some related tech.