Relationship Between Exchanges and Operators
Telephone calls must be routed across a network of multiple exchanges, potentially owned by different telephone carriers. The exchanges are all connected using trunks. Each exchange has many "neighbours", some of which are also owned by the same telephone operator, and some of which are owned by different operators. When neighbouring exchanges are owned by different operators, they are known as interconnect points.
This means that there is really only one virtual network in the world that enables any phone to call any other phone. This virtual network comprises many interconnected operators, each with their own exchange network. Every operator can then route calls directly to their own customers, or pass them on to another operator if the call is not for one of their customers.
The PSTN is not a fully meshed network with every operator connected to every other - that would be both impractical and inefficient. Therefore calls may be routed through intermediate operator networks before they reach their final destination. One of the major problems in PSTN routing is determining how to route this call in the most cost effective and timely manner.
Read more about this topic: Routing In The PSTN
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