The architecture of business telecom is really flexible. Customer PBXes have wide latitude to make their own routing decisions, and numbers are shared across arbitrarily complex multi-site, multi-provider topologies.
What your neighborhood grocery store buys from the phone company is just a pool of voice circuits, probably fewer than its extensions and even its DIDs. There is no simple circuit-number mapping like in residential telecom.
Designing federated AuthN/AuthZ for this is not simple; getting it rolled out across all the participating equipment (with many owners and service lifetimes of 15+ years) is basically never gonna happen.
What your neighborhood grocery store buys from the phone company is just a pool of voice circuits, probably fewer than its extensions and even its DIDs. There is no simple circuit-number mapping like in residential telecom.
Designing federated AuthN/AuthZ for this is not simple; getting it rolled out across all the participating equipment (with many owners and service lifetimes of 15+ years) is basically never gonna happen.