Redmond, Wash.—Microsoft Corp. has launched a click-to-call service for search ads appearing on its Bing search engine, using the Skype phone service the company acquired in 2011. The company's Call Extensions service, also available with Yahoo Inc. search ads, provides a phone number alongside search ads on PCs, tablets and smartphones, Microsoft said in a statement. Customers can click the phone number to place a call, with advertisers charged on a cost-per-click basis. The service provides reports, including length of time on the calls, impressions and resulting revenue. Marketers can provide values to these leads to optimize their search-ad budgets, Microsoft said.