Ad network operator Marchex is acquiring pay-per-call firm VoiceStar, reports MediaWeek.

VoiceStar operates a pay-per-call program that lets small businesses advertise online, even if they lack a robust web presence. Those ads generate phone calls to the business, which can be tracked.

Marchex feels VoiceStar's ads will be able to fold easily into existing operations. The firm offers search and other offerings, all with a local focus. VoiceStar's pay-per-call ads are seen as a logical extension of that focus.

Pay-per-call is estimated to be a $4 billion business in the next two years.