Steve6443 don't subscribe to the theory of someone else hijacking the plane by overpowering the crew.
Agree, the pilots could have got a call out or just held open the mic. Also they would ave to be pretty quick to disable all the comms, especially the SAT phones to stop anyone in the cabin making a call on the seat telephone.
They would also have to be knowledgeable of the DATA comms system and disable the ADS CPDLC SAT HF and VHF data link.
The pilots could do this in a matter of seconds, also best to switch off the ADS CPDLC and transponder when transiting from one sector to another. This would not arouse any immediate suspicion as one sector would think the other was still in 'control'.
It would take several minutes for the controllers to be aware something was amiss and by that time the plane would be well out of the area and so difficult to spot on primary radar. It would also take time to alert an operator of a primary radar set (military), that the aircraft was lost.