I guarantee that Verizon isn't using anyone else's network. They have even embarked on a multibillion dollar capital plan to build their own fiber networks in the top 100 US cities so they can use their own infrastructure to connect small cells, 5G, and business customers. They do not like using others' infrastructure. AT&T and T-Mobile do lease physical infrastructure from others but they would never in a million years give another company access to their networks for IP traffic.

Bandwidth operates in a completely different realm. They provide cloud-based IP comms services (the industry term is communications platform as a service). They compete with the likes of Ringcentral, voip.ms, and others. Nothing to do with how the big 3 cellular carriers route their calls or data. I had actually never heard of them before. They claim they own their own tier 1 network, which probably means they have IRUs on fiber owned by the big companies.