Link: http://zfoneproject.com

While i was around enigmail and pgp i ended up with zphone.
Worth to check.

ZfoneTM is a new secure VoIP phone software product which lets you make encrypted phone calls over the Internet. Its principal designer is Phil Zimmermann, the creator of PGP, the most widely used email encryption software in the world. Zfone uses a new protocol called ZRTP, which has a better architecture than the other approaches to secure VoIP.

"Zfone brings badly needed VoIP encryption
to the masses, in an open, easy-to-use,
and secure way. It's about time."
- Bruce Schneier

you may also want to read about regular cellphone encrypted txt/sms