You have secrets you need to talk about on mobile phones? You better use a CryptoPhone to keep the secrets secure.

German GSMK offers mobile phones from HTC that run a special software that encrypts calls with 4096 bit Diffie-Hellman key exchange and with SHA256 hash function AES 256 and Twofish between Cryptophones.
Security does not come cheap. The Cryptophone G10i sells for about $2080. The clamshell phone is a quad-band GSM phone with EDGE and GPRS support.
GSMK also offers the PDA phone CryptoPhone 220, which is even more expensive.
Heise (German) that the CryptoPhone sell pretty well. GSMK is not talking about details though.
More details on GSMK site.