Cell phones want work at altitude. And they went from 37,000 feet to 15,000 feet extremely quickly which no-one would be in a mind state to make a call.
I’m thinking terrorism. Explosion and they dropped out of the sky.
Sad
Rapid decompression at altitude the TUC would be measured in seconds, and if explosion the confusion and chaos might have prevented passengers and crew from donning functional O-2 masks.