The architecture of business telecom is really flexible. Customer PBXes have wide latitude to make their own routing decisions, and numbers are shared across ar

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.