I just got a 502 Bad Gateway trying to look at their homepage to find out what it is they do.

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).

That must be why 99% of the SMS spam I receive comes back to Bandwidth as the carrier. On the bright side, I haven't gotten any spam in the last hour or so.

Even big enough ones, like RingCentral.

RCLEC (RingCentral's CLEC) does not seem to host the majority of RingCentral's numbers despite being fully capable of doing so, RingCentral seem to sprinkle their numbers across Onvoy & Bandwidth.com if memory serves.

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.

Phone services for other carriers and companies, like DIDs (phone numbers), SIP trunks, SMS, and 911.

RingCentral, Nextiva, Microsoft Teams Voice (fully dependent on Bandwidth.com), Google Voice, Vonage (for a subset of numbers), and most wholesalers have some or all phone numbers on Bandwidth.com