This is also what I suspect - she used her phone to take videos/photos but kept it in airplane mode. She could have possibly sent the snaps / vids only once she connected to Wi-Fi to save on data roaming charges (that’s what I do traveling internationally - I only use data if I absolutely need to). If her phone was in airplane mode, that would provide an alternative reason why calls went direct to VM. The phone may have been fully charged, but in airplane mode.
@mark1969 - do you know if JF was sharing photos and videos real time with her family? Or were they shared in batches at a different time?
On a related note, ifJF had her phone in airplane mode, the cellular radio in her phone would have been disabled, so her phone wouldn’t have been pinging cell towers along their journey.