I was wondering today whether Office Communicator offered any integration with web pages to enable calling a phone number on a web page. It turns out it does (as the Communicator team explains on their blog) via the tel: protocol.
I confirmed this by inspecting the registry on a computer with the Office Communicator 2007 client installed. Sure enough, it is registered for tel: (and also callto:).
If your page is just for your intranet, you can probably get away with the local number, but the RFC strongly recommends you use full "international form" – that way the number should be callable from anywhere. eg.
Telephone: <a href="tel:+3585551234567">+358-555-1234567</a>
I don't have Communicator installed on my home PC, but I do have Skype. A quick check of the registry confirmed that Skype has registered as a handler for callto: but there is no tel:.