Scenario

A typical application is like that:

Home phone1 - (PSTN) - PC1 - (Internet) - PC2 - (PSTN) - Home phone2

1. Home Telephone1 make a calls to PC1 phone number (using PSTN line, I mean classic telephone line).
2. PC1 answer to it.
3. Home telephone1 must tell PC1 what gateway use (PC2 in this case) by giving the IP address (from DTMF keyboard) and/or what number call (in this case Home telephone2).
4. After that PC1 will start to make an H323 call to PC2, then it will pass Home telephone2 to PC2 to make it call it throught PSTN line.
5. Home telephone2 answers to call and communication between Home telephone1 and Home telephone2 begins.

What can be changed in this configuration?

1. You may use a PBX to select many lines to access many PC1 gateway (for example one to call within your state, one to go accross Europe, and so on...): typically you don't have to change this, cause cost is always the same.
2. You can select (after called your PC1 gateway) every PC2 you want (for example a PC2 living in England to call an English person so that you'd pay only intra-country costs).

So your decision will be taken considering PSTN line costs. In fact what VoIP does is the convert this:

Home Telephone1 --- (PSTN) --- Home Telephone2
             PSTN great distance calling cost

into this:

      Home Telephone1 --- (PSTN) --- PC1   +
      PC2 ---- (PSTN) --- Home Telephone2  =
      --------------------------------------
          2 PSTN short distance calling costs

To save money you need that:

2 PSTN short distance calling costs < PSTN great distance calling cost 

Typically "short distance calling" refers to a "city cal" while "great distance calling" can be an "intercontinental call"!