If my post came across as condescending, please forgive me. I really was just having a little fun, and in fact Reeses intuitive explanation was OK, up to a point, which I was also trying to establish.
my cellphones auto locater signal amplified itself through the speakers of our Yamaha DGX 500 digital piano which was on in the other room.
OK, but does your phone turn on your TV, or your kids' toys, or flash the lights in the house, too? Or, conversely, does your phone pick up AM radio stations? My point is that I don't doubt there's electromagnetic noise from your phone and it can affect other devices -- one of the reasons the various bands are sequestered and that internal circuits have to obey FCC specs for RF noise is because of the recognition of that fact. But that doesn't mean they can affect everything (or conversely that everything affects them) and in particular it doesn't mean they can affect human beings.
I'd guess there is some resonance in a circuit at that frequency your piano responds to, effectively "tuning it in." Electromagnetic fields produce induction currents. EM fields can affect humans (ask the Hiroshima survivors) but we don't have a mechanism whereby fields in this range at this intensity are capable of it.
If we could all converse at this level one thing the cellphone companies (gangsters really) couldn't get away with is limiting the bandwidth so much you can't understand the transmission w/o great effort.
It's almost as if we have to use another portion of ours brains to fill in the blanks left in the conversations at 8-13 Kbits per second they allow us to use for hundreds of dollars per month.
With only a little more effort on their part the cellphone could be almost CD quality.
But, most of our caveman brethren are content with fire on a stick and have never seen a Bic..