VoIP providers generally have to connect to the PSTN to accept incoming calls (origination) and to outgoing calls (termination). Termination seems to be an easy thing to provide: The provider simply leases a few PRIs and a channel bank at the datacenter of their choice. However, call origination is more of a mystery to me. How are VoIP providers able to offer DIDs in such large number of rate centers? They can’t possibly have their own equipment in all these areas.
Simple answer is that DID numbers, from various locations, can easily be routed to terminate on a set of large PRI/T1-T3 connections, that may or may not be directly served by the Central Office (CO) that handles the city prefixes in question.