I personally like SFLPhone [1]. It's developed by the fine folks from Savoir-faire Linux [2] and supports encryption. Here is a guide how to configure it to encrypt traffic between the client and an Asterisk server [3].

[1] http://sflphone.org/ [2] http://www.savoirfairelinux.com/en/ [3] https://projects.savoirfairelinux.com/projects/sflphone/wiki...