Using the current pay-per-click model, Google and other sites auction the right for a company’s ad to appear next to search results when somebody enters keyword and the advertiser pays the search engine when someone clicks on the ad link. But with the pay-per-call model, users are instead directed to call the advertiser, who is billed for each phone referral.

Google is said to be testing a variant on the model in which users would click on a phone icon and type their number into a box. Google then dials the user, who hears ringing until the merchant answers. The service is free for callers even on long-distance calls, and it promises not to divulge the caller’s number to anyone.

Meanwhile MSN is following in Google’s footsteps by testing a service that would enable people to place classified ads similar to Google Base. Microsoft has been testing it for a couple of weeks and has given it the code name Fremont, but no launch date has been put in place.