My guess is that the third party SIP client is a little more customizable for the user than perhaps CIPC may be.  What about using downloadable SSLVPN connectivity as opposed to an installed IPSec client?  Would that work for the customer?

Not quite what you're looking for, I'm sure...  but, definitely needs to be mentioned. Everyone knows what CAPTCHA is, and most people understand how it's supposed to reduce the deluge of spam and potentially other malicious use of Internet bandwidth...