For companies like SIPStation, Voip.ms, VoIP Innovations, Vitelity (to some extent) and even ones like myself and others here are aggregates. We have numerous peers with upstream providers and in some cases they don’t offer SMS with numbers, that to the upstream are “off-net”. So I use Bandwidth but in some areas Bandwidth, themselves, don’t have a footprint so they have peered with another LEC and can’t offer their services to those DIDs because those DIDs aren’t on their network and equipment.
So basically when the upstream LEC is handing out DIDs from one of there non-footprint COs and partners.