Asterisk and SSL

If your Asterisk server is internet visible then you really want to look to secure it.  Have a look at this document for details on setting your self up as a signing authority. This allows you to create a certificate which can then be signed and accepted by your users software.

http://www.rho.cc/index.php/faq/asteriskc2d/50-asteriskc2d-iphone/47-iphone-and-ssl-certificates

The above method is free, your other choice is to buy a certificate from a recognised signing authority.