Click To Call is one of those features that Google freely admits to working on -- but is still considered an experiment. This feature provides more direct communication between users and advertisers by creating a direct phone to phone connection between the two parties when an icon in the advertisement is clicked.
Last week I blogged about a new entry in the robots.txt file that disallows the /call directory -- since then the page was removed and brought back to life in a slightly altered form. By visiting the disallowed URL, we can still see the useless XML document telling us there has been an error, but NateDawg from the Google Blogoscoped forums noticed tonight something interesting. The headers for this site now mention "ctcserver" -- a word that one can only speculate means "Click to Call Server".