How does VOIP works in this sense...?

Some companies offer VOIP product- you can have a ph number in another country so people from that country can call you locally and the call will be transfer to your ph of choice here in States. I think that since this service is VOIP they should offer the service in any city of that country but they don't. I would like to learn why not.. I used to work with telecom and I remember that sometimes we need to connect with cities that have hubs- but this was pre-VOIP era.... What's the deal here?How does VOIP works in this sense...?
the other companies would have to change their infastructure to do so. VOIP stands for ';voice over internet protocol';. these voip companies are using bandwidth and phone line rental to do buisness. the old buisnesses are doing it with what they already have, phone lines. the old buisnesses feel they have no reason to change. it would cost them a ton of money, even if it were to help move along with the times. this happens alot in america, which is the reason we don't have blazing fast internet, everyone driving using ethanol fuel, everyone with high definition televisions and cell phones like other countries.How does VOIP works in this sense...?
Phones work over a bandwith of about 3500 cycles, even though natural voice goes to about 20,000 cycles. Ph. co.'s stack the voice channels and trunk lines at much higher frequencies transmit these. Just like a dsl line a voice channel needs a dedicated modem for each ph line. The difference is interfacing with the internet and providers. Lots of switching going on there...