Phones should be secure as possible, if you need an unwalled garden use a computer.

Security is great and all, but walled gardens are just another lackluster attempt at security through obscurity. It never works. Never has, never will.

Telegram's MTProto encryption protocol is a walled garden, and I would be shocked if the NSA weren't poking holes in it for fun. Signal's open Axolotl double ratchet, however- that shit is ridiculously secure. Walled gardens are moot.