Design a Global One-to-One Voice Calling System

Design a global one-to-one voice-calling system for millions of registered mobile users. A user can call another registered user, the recipient receives an incoming-call notification, accepts or rejects, and accepted calls support real-time audio.

Constraints & Assumptions

• Distinguish account/device registration, call signaling, incoming-call delivery, and media transport.
• Describe who sends each request, what identifiers and state it carries, and how the next step follows the response.
• Clarify multi-device behavior and offline/background delivery; user count alone does not specify concurrent calls.
• Keep user/device lifecycle responsibilities clear even if an early deployment co-locates services.

Clarifying Questions to Ask

• Can one user be signed in on multiple devices, and should all of them ring?
• What is the policy for busy users, unanswered calls, and missed-call notifications?
• Is peer-to-peer media acceptable, and what relay coverage is required for restricted networks?
• Which identity and privacy checks govern caller information shown to the recipient?

Part 1 — Registration and call setup

Describe user/device records and the caller-to-recipient signaling sequence.

What This Part Should Cover

• When device IDs, push tokens, and user/profile data are stored and updated.
• Authenticated create-call requests and incoming-call notifications.
• The source of caller display information and checks against spoofed client fields.

Part 2 — Accept, reject, and media

Describe state transitions and how accepted participants establish an audio path.

What This Part Should Cover

• Call IDs, idempotent state transitions, competing accepts, timeouts, and cancellation.
• Offer/answer and connectivity negotiation, with relay fallback.
• Separation of signaling from real-time media and protection of media/session credentials.

Part 3 — Global operation

Explain how signaling and media resources scale and how the system handles disconnection.

What This Part Should Cover

• Regional routing, call ownership, active-connection state, and durable summaries.
• Latency, packet loss, jitter, connectivity, and setup-success monitoring.
• Reconnect and hang-up behavior without leaving inconsistent active-call state.