People who have a PSTN phone (or a DID) can map that phone number to a SIP URI (for free at , but no new registrations - project is abandoned)-- that mapping is called ENUM. Whenever someone calls that PSTN phone number from a SIP client or proxy that checks ENUM, the call goes to the mapped SIP URI (rather than via PSTN) and is free.
With SIP Broker, if an ENUM entry is not found, then the call is sent back to the SIP adapter, which then places the call in exactly the same way as if SIP Broker had never been involved. So if a SIP adapter is registered with a prepaid SIP-to-PSTN gateway (like SIPphone or OZtell), then the call will continue regardless if SIP Broker was able to find an ENUM entry.
Dialing a direct URI can be difficult since phone keypads do not usually include characters other than numbers. SIPbroker maps 'fwd.pulver.com' to '*393', so you would just dial '*393613'.