Online advertising firm Marchex Inc. has entered the mobile market to offer call-based ad services to key mobile partners.

The Seattle company, whose ad platform offers search and call-based marketing, has agreed to offer call-based ad services to mobile ad networks AdMob, Ringleader Digital and 4Info.

"We've been looking at the mobile market with increasing interest over the last few years," said John Keister, president and chief operating officer of Marchex.

"As the mobile advertising business ramps up, there's a keenness to track the calls and the ads and have reporting such that where the calls are coming from, the quality of the calls, the length of the calls," he said. "Metrics in this area of are extremely trackable, for that's the value of call advertising."

Marchex's VoiceStar subsidiary will offer call tracking services to the three mobile ad networks. It will act as a white label service behind the ad network.

"We think the mobile device is really built for call advertising, more so than display advertising," Mr. Keister said.

"We think the mobile platform will be as important in the next few years as any other platform for call ads," he said.

The VoiceStar call tracking will let the networks track the calls generated by ads on their network and gauge which ads delivered the calls.

VoiceStar will also be able to track and report information such as duration, time of day and geographic location of callers, as well as record the calls.

This information will be available through the Marchex reporting interface.

"What this means is that the reporting they will see will be much richer and they will have many more tools at their disposal to quantify the value of their advertising," Mr. Keister said.

Also, Marchex will offer pay-per-phone-call services to AdMob, San Mateo, CA. AdMob can now bill advertisers for the calls they deliver.

"We're finding that click-to-call is a powerful ad unit for mobile," said Jason Spero, vice president of marketing at AdMob.

"AdMob was early to introduce this and has been perfecting this for advertisers," he said. "Marchex enables us precise tracking on behalf of advertisers."