asterisk users - Jun 2004 - Newbie question about dialling PSTN numbers from SIP clients

Hi list,

I have an asterisk server with a Zap 4 port FXO card connected to the PSTN,
all of the clients are SIP softphones (I have tried LIPZ4 and kphone). I
have successfully configured asterisk to route incoming calls from the PSTN
to an extention that is a SIP softphone and the user can answer the call.
Question I have is how do I call a PSTN address or extention number from a
SIP softphone if the number to dial needs to be a SIP address in the client
software? I assume that you need to encode the number or extention into a
SIP address, specifying the IP address (or hostname) of the asterisk server
as the recipient. Is this correct? And how do I encode the addresses? Is
there anything special I need to configure on the asterisk side to act as a
media gateway (over and above the basic config I found in the document "How
to configure asterisk for LIPZ4")?

Thanks for the help.
Dave