Depnds on the equipment that you have.
On a quintum, PSTN to VOIP is done trought Static Routes.
VOIP to PSTN is done setting up PSTNTG1. or PBXTG1
If you are working on a cisco ,they are called dial peers VOIP (From a port to VOIP), or dial peers POTS (from voip to a port).
Very basic setup on both equipments, you basically tie together one or more I.P. ‘s and phone numbers.