If there was indication that something was awry the passengers would have had plenty of time before reaching the open ocean to use various forms of communication.

It is very unlikely (but has occasionally happened) that a cellphone would get reception at 35,000 feet.

The plane reportedly flew under the radar after turning away from Kuala Lumpur.

source?

This is a good question. What are the odds that someone on the flight had a sat cell? If so, would it have worked in those conditions?