SIP Trunk 503 Service Unavailable during JMeter burst test

How come this setting causes immediate 503 Service Unavailable errors on the SIP trunk endpoint when hitting 20 concurrent INVITEs from Singapore? The Genesys Cloud platform_api reports the calls as ‘failed’ while the underlying SIP server logs show no traffic. Running JMeter 5.6.2 against api.mypurecloud.com. SIP Trunk Config Docs