The article doesn't really say what hackers had access to, but it sounds like they had full control over their phones. There is a lot bigger story here and I'd love to read a post-mortem in a few months. Also, WhatsApp is such an obvious target for a state actor. I saw several articles of the last year that mentioned Jared Kushner using Whatsapp so I assume a lot of government folks use it for off the books "encrypte…
Your best bet is to use something obscure so you cannot easily be targeted or not being connected to the same net.
At the Israeli army we had phones that ran completely separate software stacks and talked to a different network for this. Nothing was on the internet.