I'm convinced that if website authors ever get their collective acts together and provide clickable standard URIs, combining with my mobile phone usage patterns I may never touch the dialpad again (except to dial 911). The firefox solution is great because it requires a single click. However, the majority of corporate users aren't on firefox and those that are often don't install greasemonkey. This solution would have been a heck of a lot more elegant if not for a bug in IE that doesn't properly support redirect to non-http URLs. Props to the Microsoft guy who emailed me back and explained it to me. I hope you fix it some day and users will be able to avoid that extra click.
Web Dialer Accelerator
Check the 'Make this my default' box and then click Add. Checking this box will minimize the number of actions required to dial a number.
Now that the accelerator is installed, go to any website with a phone number like Google Maps. Select a phone number. Don't worry if you miss some of the formatting in your clipboard. Most good PC VoIP apps will correct the number for you. Once a number is selected, the accelerator hover icon will appear. Click it (or right click) and hover over Web Dialer. A link will appear. Click that link and your VoIP app will dial the number. If instead of hovering you click on the Web Dialer item, a page will load with the same link.