WebRTC 503 during JMeter concurrent softphone login test

Looking for advice on handling 503 Service Unavailable when simulating high-volume agent logins via the WebRTC softphone.

Environment: Genesys Cloud US1
Tool: JMeter 5.6
Endpoint: /api/v2/platform/user/login

The test sends 50 concurrent requests. Most fail with 503 after 2 seconds. Is this a platform rate limit or a WebSocket handshake timeout? Need to validate capacity before peak.