Let's say I'm a not entirely small company, with employees with phones on their desks who call customers and have direct numbers to be called. When they call out, the caller id should be their desk number so the people they're calling know it's them and answer the phone.
But the carrier that handles inbound calling charges an arm and a leg for outbound, so I'd like to use another carrier for outbound; I don't want to change my inbound carrier.
In that case there should still be some similarity among the numbers (area code, exchange, etc), and it wouldn't be difficult to tell the outgoing provider that you're going to be using a particular block of numbers. At a minimum, the outgoing calls shouldn't have numbers from different area codes every day.
But the carrier that handles inbound calling charges an arm and a leg for outbound, so I'd like to use another carrier for outbound; I don't want to change my inbound carrier.