Here is a map of a relevant part of INMARSAT coverage and the red circle.
The red circle are the points which see the Indian Ocean IOR at 40 degree altitude.
The pink circle are places which can just see the pacific POR above the horizon (I think about 8 degrees). So if you are east of the pink line and ping then POR will hear you. (This did not happen to MH370 at 8:11am).
The green circle is the limit for the Atlantic AOR-E. So if you are west of that then AOR-E will hear you.
Hence, if you ping on the red line and POR does not hear you as well as IOR then you can not be in the South China Sea.