I wanna have voip services as a new service to my webhosting customers

I'm a newbie, I know companies in US that provide unlimited calls with a monthly fee, I wanna know how can they do that? in my country we have phone to phone cards available which can be used to call US/Canada with a low rate/min only
how can I provide unlimited calling services in my country with the ability to call other countries?

Look at it this way: there is no unlimited, there is only creative marketing and economy of scale.

there is no unlimited, there is only creative marketing and economy of scale.
normally some restrictions applies.

Do you only want to offer calling services in Usa/Canada ? Or other you want to open all international calls as well ?

Just to clarify you on the unlimited plans infact none of them are unlimited most of them have caps upto 2000 mins a month. So for $20 u can give 2000 mins for 1c to USA/Canada.

Calling to international destionations will be much more expensive. But still you can make something out.

I'm not US/Canada based, I want to offer unlimited local calling plans and also international plans, I've found a voip service that's not in my country but offers unlimited (per call) plan to my country but they don't offer reseller services, the rates of my country are about $0.06 to $0.16 depends on the city

I read tutorials on how to start a voip
but they're not really clear, OK
So let's say I've got all the hardware, software and bandwidth needed, where do the rates come from? do I have to work with telecom? I don't get it

OK, I have this another question
voip connects 2 computers together and the second computer is connected to PSTN lines
so if we call US from UK then we can call at local rates in US

but this explanation doesn't make sense
then if it's voip, then how can voip services offer local cheap rates, even cheaper than PSTN 2 PSTN calls in the same country it's located
and why some countries rates are higher?

thanks guys for your help :-)