In a recent post o his blog Simon Harris talks about how you can make phone calls and send SMS messages from your PHP applications (well, sort of). He shows how to trigger phone calls and SMS on certain phones by way of an HTML page.
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.