Malaysia and Thailand are relatively densely populated. Coverage is very heavy along both coasts of the countries, but much less complete inland.

This is a coverage map for one of 4 Malaysian mobile operators.

http://opensignal.com/networks/malaysia/my-celcom-coverage

Thanks. I haven't been following this closely enough to have an idea of the (presumed) flight path and thought the land travelled over was Indonesia, which seems to be not so dense on that western island.