Over the past few months, Skype has been testing click-to-call ads on the Web, partnering with local-business directories who pay for the calls.

Skype is set to announce the official roll-out of its click-to-call advertising service with European Directories, one of the largest local business directory services in Europe.

Turning phone numbers found on the Web into ads could help Skype open up a significant new line of revenue.

But already Skype is playing catch-up against younger rivals. Yext, one of this year’s TechCrunch50 crop which just raised $25 million in new funding, has been turning online directory listings into click-to-call leads for years.

Photo by TechCrunch.