Cell towers are too low to use them to detect a stealth plane passing in between their antennas and anything on the ground, but satellites could be used. We're surrounded by radio signals originating from the sky, so at least in theory the right equipment could detect a stealth plane not thanks to the radio waves it reflects but the ones it doesn't let pass through itself.

It was a joke :)

Not necessarily, but I still doubt this would be successful in detecting a stealth plane.