You shouldn't use a phone for anything sensitive, but if you absolutely must, there are encryption options available. Look into RedPhone and TextSecure, both developed by a well known cryptoanarchist / hacker*. Make sure you root your phone, and remember that metadata like the numbers you call/text and the time of each communication will still be available to the carrier and LE. As the saying goes, "metadata in aggregate is content." You're better off using desktop communication tools, email + PGP, softphones with ZRTP, and running everything over anonymity networks. * https://en.wikipedia.org/wiki/Whisper_Systems