This is quite interesting.
Mig33, the VoIP/IM over Mobile player, has launched a ToolBar for Internet Explorer that contains a phone client for contacting your Mig33 friends on their mobiles.
http://www.mig33.com/what_is_mig33_CheapVoip.php
About halfway down on the right hand side of the page.
What has this to do with click to call? Well, take a toolbar like that and have it parse the web page currently being browsed for phone numbers. Inject a little "toolbar://" tag, registered to the toolbar in the Windows subsystem at install time, and you have a neat little system.
And one that you could probably charge vending webmasters for on a per-lead basis.
Dean