If people are not at work, they may not have the money to keep using the cellphones, or they may not be using them for work uses.
I wouldn't assume it's due to a massive death toll.
As an aside, I was driving a cab part time a few years ago. I picked up a Chinese student, and we chatted a bit about China etc. He made the comment "It's not North Korea!" which implies he'd either picked up enough from the our local press about North Korean repression, or even in China, the North Koreans have a reputation for a dictatorial state, compared to their own.