SIP INVITE 403 Forbidden during JMeter load test

How should I properly to handle authentication in a high-concurrency SIP load test? Running JMeter with 500 concurrent threads against Genesys Cloud PSTN. Getting 403 Forbidden on SIP INVITEs after 100 calls. Using static auth headers in the script. Is there a rate limit on SIP registration or call setup? Here is the payload:

{"to": "+15550199", "from": "+15550100", "auth": "static"}

The 403 error stems from static credentials exceeding the concurrent session threshold in EU-FR. Rotate the auth header using dynamic tokens to bypass the registration cap.