WhatsApp does secure voice
So anyone who wants to make a call that their mobe network can't and over to the police just needs one of the most popular apps around.
There is a big difference between IMSI capture and listening to the call. And litening to a call from a plane where there are cell hand-off issues would be very hard.
I often wonder how the secure phones (Black Phone, General Dynamics et Al) handle the MAC address. There is no point the phone bit staying under the radar if the WiFi is broadcasting an unique number.