Design a global one-to-one voice calling system

Design voice calls between registered mobile users worldwide, at millions of users. A caller rings another user, the callee gets an incoming-call notification and accepts or rejects, and accepted calls carry real-time audio. Cover registration and presence, the signaling flow, push notifications to sleeping devices, media relay versus peer-to-peer and NAT traversal, routing calls across regions for low latency, call state and failure handling, and what you would measure.