Home telephone1 -- (PSTN) -- PC1 -- (Internet) -- PC2 -- (PSTN) -- Home telephone2
Home Telephone1 make a calls to PC1 phone number (using PSTN
line, I mean classic telephone line).
PC1 answer to it.
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).
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.
Home telephone2 answers to call and communication between Home
telephone1 and Home telephone2 begins.
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.
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"!