It seems like I just found out about another way to do something with the Windows Phone 7 OS directly from the browser. I can invoke the Maps application, there is information on the browser version, there is a possibility to navigate to an item in the Zune Marketplace, you can launch the YouTube application and now there is a way to dial phone numbers through what seems to be an internal system URI scheme that is able to carry much more than simply calling somebody.
There are two ways of invoking a call - the old one, that is supported both in Mango and NoDo relies on the tel prefix and the general format is this:
The URL can be invoked directly via a WebBrowser task or a link that is present on a webpage. You will not be able to type it in IE because it will return an error - the browser doesn't support the URI scheme that is being used. For some reason, as long as the URL property is present in WebBrowserTask (allowing to pass unverified URL strings), the same input can actually result in a call.
With Mango, there appears to be a new way to invoke a call:
It doesn't seem to be working when passed through a WebBrowserTask, and it won't work if it's typed in directly, but it can certainly be a part of a webpage where a phone number is required.
The URI scheme belongs to the standard WAP specification, allowing the invocation of the OS number dialing dialog.