Every third party that processes customer data on our behalf, grouped by function: model providers, speech recognition and synthesis, telephony, retrieval, hosting, and analytics. Each entry states what it processes, where it is located, and what it retains.
The fallback chain is a reliability feature, and improving it means adding or reordering providers. Voice vendors are swapped when quality or price moves. Each change is a change to your data flow, so it is published with advance notice rather than reflected quietly after the fact.
Customers under a data processing agreement can object to a new sub-processor within the notice period. In practice the resolution is usually to pin that customer's builds to the existing provider set, which is technically possible because the routing is per-tenant rather than global.