Consider that every international flight anywhere in the world is in constant contact with someone.
When outside of radar coverage (trans-oceanic) there are position reports to be maintained if for no other reason than to keep the airline company appraised of their status in regard to scheduling.
All it takes is a push of the finger to make radio contact.
Complete radio silence indicates either complete and utter electrical failure, battery backup included (unimaginable), lack of ability by the crew to do so (dead or incapacitated), both captain and first Officer simultaneously (unlikely) or an "inside job" by one or both of the flight crew.
Consider that every international flight anywhere in the world is in constant contact with someone.
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1st rules of flying -
aviate
navigate
communicate
if there is a SHTF situation on an aircraft the last thing they do talk to a control center... That’s why we have black boxes...