Routes (to numbers)

The path from an ITSP to a destination phone number is called a (SIP) route. Often the ITSP (and major carriers and Telcos alike) has many routes it can choose from to connect the outbound call originated by its customer's SIP device. This exchange of routes minutes is a very big and complex business, and, if we include the big Telcos, is one of the major businesses on Earth.

As you can imagine, the ramifications of such a business depend on local regulations, international agreements, geopolitical situations, business alliances, economic development levels, and a thousand other factors.

In some countries and regions, origination (gathering and routing of outbound calls) and/or termination (providing PSTN gateways to inbound calls) is a legal monopoly of one or few companies; in other regions and countries, regulation requirements can set the bar of entering the business in a way that floods the market with pop and mom's shops or that makes it the exclusive preserve of companies worth billions.

Each ITSP chooses its own mix of different route providers, and there are many "meta-ITSPs" specialized in aggregating and arbitrating traffic from many different route providers and packaging it for other ITSPs that sell minutes to end customers.