SIPBroker is free because it is part of the voxalot range of services.

The PSTN to VOIP service is usually a local number or a nationwide lo rate number , but I guess the sponsors make a small amount of money for each call to pay for the service.

I can't see how you could be marketed to as you don't enter any details to use the service. I certainly have never been spammed or similar.

Give it a try.