If you are lucky enough to have Office Communicator deployed with telephony integration enabled you will have already seen the SharePoint functionality to initiate a telephone call with a document author or from the user profile page.

You can create a hyperlink in a web page or email message to initiate a call by using the tel: protocol. You can also enable this functionality in a SharePoint Contacts lists with the Data View web part. Just replace the <xsl:value-of select="@WorkPhone" /> with

Click to call: <a href="tel:{@WorkPhone}" ><xsl:value-of select="@WorkPhone" /></a>

While updating one of my Virtual Earth demos because Microsoft had turned off the older pre v6 map control API's, I decided to add this "click to call" functionality to the Office Locations contact list here http://www.wssdemo.com/demo . The map pin also allows you to initiate a call...

I will also be incorporating this into my Meeting Room demo that is about to be updated to provide maps with floor plans.

I don't know if this works with MSN Messenger or other VOIP products. Trry my number tel:+652068883209 but don't expect me to pay your phone bill (SharePoint rich text field doesn't like tel: hyperlinks :-(