Jambo is testing pay-per-call ads in the radio environment, buying up remnant spots from a newish reverse auction firm called Bid4Spots. Business categories covered under the tests include DIRECTV installers, mortgage lenders, and attorneys; and advertisers can specify such targeting factors as market, daypart, demographic and station format.
If this test and others like it go well, radio advertising may eventually migrate to a scenario where marketers, the lead-focused ones anyway, will pay only for pre-qualified calls. (link to press release)