VoIP SIP signaling system

Design a VoIP signaling system (the control plane for placing and tearing down calls) for a cloud telephony provider handling 2M registered endpoints and a peak of 5,000 call setups/sec. Signaling uses SIP. You need <250 ms post-dial delay, correct call state through network blips, and the ability to survive a signaling-node failure mid-call. Media (RTP) flows separately, focus on signaling, registration, and call-state management.