asterisk users - Aug 2007 - VoIP encryption with SIP and IAX

Dear all, I have an Asterisk server with SIP and IAX softphones clients, and I
need to encrypt the voip calls among them:

*For SIP clients I use Twinkle which implements the ZRTP/SRTP encryption
mechanism client-2-client; I read it's the better security mechanism
nowadays created by Phill Zimmerman who created PGP.

*For IAX clients I used Kiax but I don't know exactly if there is any
encryption mechanism for this protocol.

Two short questions:

1) Do you think ZRTP/SRTP is the best option to encrypt SIP voip calls ???

2) What is the best way to encrypt IAX voip calls ???

Really thanks

Alejandro