Robocalls. That said, they're likely "virtual" providers, which are in turn enabled by "Mobile Virtual Network Providers", companies that sell telco-provider-as-a-service, so that a virtual provider only needs to focus on sales, marketing, first call customer support, and legal liability but not the technical nitty gritty. I hope the FCC goes after these MVNOs enabling them next.

(source: wikipedia and I worked in a small segment of the mobile network operator market for a short while)