Required under GDPR for any controller-processor relationship involving EU personal data. Most enterprise vendors offer a pre-signed DPA on request.
Key DPA clauses: sub-processor list (annex), data flows, security commitments, breach notification timeline, audit rights, international transfer mechanism (SCCs, adequacy).
Callsy provides a DPA + annex for all paying customers. The annex (linked from the privacy policy) lists current sub-processors with data category and location.