pdivos@gmail.com wrote:
> I were always wondering how a voip system connects to "traditional"
> phone systems. Do voip providers have a contract with each phone
> provider they are connected to? Do they have separate gateways to each
> phone provider?
> And how is it possible, that calling normal phones (landline and mobile
> as well) through a voip provider is usually cheaper than calling them
> directly, sometimes even from the same network?
A VoIP operator will typically have one or more connection/interconnects
with network operators who can offer PSTN-VoIP gateways. Some VoIP
operators will interconnect via VoIP, others via TDM.
Companies who offer PSTN-VoIP gateways will interconnect with other
operators, sometimes only one, often more than one. You can either
interconnect with every single other operator (not easy) or you can buy
transit from one or more of the people you interconnect with, so for
example you send all of your mobile calls to C&W and they "pass them
on", if you don't have an interconnect with the mobile operator.
The reason they are normally cheaper is because they are paying
wholesale rates to their upstream rather then retail rates. Some will be
subsidising calls or taking a blend (i.e. offering free calls which are
chargable to them but their upstream, or by offering an amount of free
calls but charging a higher standard monthly subscription).
voip to landline: how it works 
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