VoIP becomes very interesting when you start to use PSTN lines
to call other people in the world, directly to their home telephone.
A typical application is like that:
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
- 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
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"!