I've seen frequency shifts with AM signals, i.e. a microphone cable hooked to am amp received a radio station quite well.
There's also stories of people's fillings and ROCKS received AM radio signals.
However I don't see how FM or a digital signal could be decoded by an AM device.
Old cell phone systems were FM, I'm pretty sure.
So it's odd you could have heard the message.
But never say never! All it takes it the right configuration of metal and just about any signal can do neat tricks.