Companies like Google scan their e-mail users’ in-boxes to deliver ads related to those messages. Will people be as willing to let a company listen in on their phone conversations to do the same?

Pudding Media, a start-up based in San Jose, Calif., is introducing an Internet phone service today that will be supported by advertising related to what people are talking about in their calls. The Web-based phone service is similar to Skype’s online service — consumers plug a headset and a microphone into their computers, dial any phone number and chat away. But unlike Internet phone services that charge by the length of the calls, Pudding Media offers calling without any toll charges.

The trade-off is that Pudding Media is eavesdropping on phone calls in order to display ads on the screen that are related to the conversation. Voice recognition software monitors the calls, selects ads based on what it hears and pushes the ads to the subscriber’s computer screen while he or she is still talking.

A conversation about movies, for example, will elicit movie reviews and ads for new films that the caller will see during the conversation. Pudding Media is working on a way to e-mail the ads and other content to the person on the other end of the call, or to show it on that person’s cellphone screen.