Lack of cell towers in the middle of nowhere maybe?
Mountains? I lose the signal on my cell phone while driving through low foothills.
The above said, commercial airlines today offer phone service right from the seat your sitting in. So why didn't someone pick up one of the many phones attached to a seatback and let someone what was happening?
That's an interesting question, considering the amount of time anyone would have had to do so. So, could the pilot alone in the cockpit have turn off those phones? If so, and he did, there might be telephone logs showing a dropped carrier. Shutting down the onboard phones might indicate the act was intentional, not medically related.
I'm not familar with the onboard phone systems used in some aircraft, but if a pilot didn't have the ability to shut it down, even if by pulling the circuit breaker, I would be very much surprised.