WURFL, The Wireless Universal Resource File (WURFL) is an open source project which collects information about all of the different mobile devices in use. It is constantly being updated, so as long as you keep your WURFL definitions up-to-date you don't have to worry about your detection scripts not recognising new devices. By querying a WURFL database with your visitor's User Agent string, you can not only determine whether the device they are using is a mobile device, but whether it has a touch screen, can make phone calls, is a tablet (iPad) and more.

They include a snippet of code that uses the Tera-WURFL service and a call to getCapabilitiesFromAgent to see what kind of client is making the requests and what it supports.

Okay, so you can't really make phone calls and send SMS messages using only HTML; that would be silly. However, if you are developing web sites and web applications for mobile handsets, you can take advantage of some features in XHTML Mobile Profile which make it easy for a user to call a number without typing that number in. You can also use the same mechanism to trigger - on the user's handset - an SMS or MMS dialog with the intended recipient's number and the message content prepopulated.

He includes three examples if it in action (using PHP to render the page) - creating a link to make a call from the HTML, sending an SMS if the text inside a link and doing the same with a MMS message. He uses the WURFL project to make it all work and code snippets are included.