Voylent (formerly Cellfire) isn't related to Zfone (which encrypts voice-over-IP communications), but essentially it performs the same basic task except that it works with cell phones. The current beta release of Voylent supports Symbian OS and requires the use of Circuit Switched Data (CSD) calls. Voylent currently works with certain Nokia phones (see the FAQ ) and can be downloaded and installed by visiting voylent.com in your cell phones Web browser. 

There is a PDF document available that explains in some amount of detail the encryption technologies used by Voylent. In summary, it uses 256-bit AES and Twofish along with 4096-bit Diffie Helmann and RSA key exchanges, as well as MD5 and SHA512 hashes.

Check it out!