In what sort of scenario are "authorities" going to be able to (or even want to) brick your phone in a timely manner, where a need to communicate is critical? In any SHTF situation, the whole (or localized) cellular infrastructure could be shut down - it's the height of hubris to think they'd be targeting you individually, and that you're the key to preventing the Illuminati from taking over. And, even if they were targeting you, the next person you run into would let you make a call on their unlimited minute plan if you simply said it was an "emergency."
A tin foil hat is much less hassle. But if you really think they're all out to get you, you should probably start carrying a firearm.