Philadelphia-based pay-per-call firm Voicestar has preened its assets for investor scrutiny, reports The Browser.

Voicestar does not operate or sell ads directly, acting instead as a white-label technology distributor: It sells its platform to publishers, who sell the ads themselves. Ads are charged to the advertiser only when a phone call is made to one of the custom numbers reflected on them.

The company currently has over 200 publishers who use the system. Analysts have made no conclusions about how much Voicestar could go for if indeed it winds up selling, but recent deals in the online world may make it hard for the company to turn down upfront cash.

The pay-per-call market is, according to a recent report, speculated to grow to $3.7 billion by 2010. The service is popular among smaller, local businesses who don't have much of a web presence but still want to take advantage of pay-per-performance ad models.