Bandwidth is a major wholesaling VoIP provider. If you're using a VoIP or other non-traditional phone service provider that isn't big enough to directly interconnect with incumbent carriers, they likely work with Bandwidth (and possibly others).
Makes for some really fun port out requests, when a client with 6 numbers has those numbers sprinkled across 3 or 4 different underlying telecoms despite RingCentral generating one unified bill for their service.