Definition
UCaaS is a ready-made cloud phone, meeting and team-chat service for employees; CPaaS is a set of APIs developers use to build communications into their own apps and customer journeys.
UCaaS (Unified Communications as a Service) replaces the office PBX: extensions, video meetings, internal chat and voicemail, sold per user. Its focus is internal collaboration.
CPaaS (Communications Platform as a Service) is programmable: SMS, WhatsApp, voice and verification APIs priced by usage. Its focus is communication with customers, automated from business systems.
The categories overlap at the edges: UCaaS vendors add APIs, and CPaaS vendors offer contact-centre and inbox products. Contact-centre platforms (CCaaS) sit between the two.
Choose UCaaS to equip staff with phones and meetings; choose CPaaS to send OTPs, notifications and campaigns, or to run customer conversations across channels.