Outbound Predictive Dialer drops calls at 50 concurrent sessions

I can’t seem to figure out why the Genesys Cloud outbound predictive dialer fails to connect calls when JMeter 5.6 simulates 50 concurrent sessions in US1.

The API returns a 503 Service Unavailable error immediately after the dialing pattern initiates, while single-thread tests work perfectly.

HTTP/1.1 503 Service Unavailable
Content-Type: application/json

Is there a hard limit on concurrent outbound call attempts for our current tier configuration?