If you want to run an experiment for the sake of science, take the battery out of a cell phone and call it from another phone.
If the passengers followed the instructions of the plane crew and put their phones in “airplane mode” (everyone does that, right?), then the last place the phone would have registered was where the plane took off. When the relatives tried to call, the phone would be paged where it last registered and no response would be received. They would hear a ringing tone until it timed out and which point it would go to voice mail or an announcement that the call could not be completed.
If the passengers left their phones on, the phone would be paged in the place where the phone was last able to registered with a compatible cell site. Again, the phone would be paged there and presumably no response would be received. Proceed as above.
Phones can send out a “powerdown” message. If the phone were catastrophically destroyed, presumably there would be no opportunity to do so. If the phone does send out such a message and the cell carrier’s system chooses to use it, then theoretically the call could be sent to voice mail immediately.
But I guess the main point is just because the callers are hearing a ringing tone when they dial the numbers, that is no proof that the phone itself is ringing. The ringing sound you hear is just a recording generated by the cell phone carrier’s tone and announcements module to reassure the caller that they should wait on the line while an attempt is made to connect the call. It is not a sound coming from (or being sent to) the actual phone.