SIP Trunk Registration Flapping at 500 Concurrent Calls in US1

Voice & Telephony
1

Why does this config cause immediate 403 Forbidden errors when we push SIP trunk concurrency beyond 400 sessions?

403 Forbidden: SIP Registration Failed - Unauthorized IP or Invalid Credentials

We are running capacity validation tests for a new BYOC trunk configuration in the US1 region. The goal is to determine the maximum stable concurrent call volume before registration stability degrades. We are using JMeter 5.6 to simulate SIP REGISTER requests and subsequent INVITE flows.

Environment Details:

  • Genesys Cloud Region: US1
  • JMeter Version: 5.6.2
  • Test Script: Custom SIP plugin simulating 500 concurrent agents.
  • Trunk Type: BYOC (Bring Your Own Carrier)
  • Carrier: Bandwidth.com

Observations:
At 100 concurrent calls, registration is stable. At 300, we see minor latency spikes (200ms). However, at 400+ concurrent sessions, the Genesys Cloud edge begins rejecting new REGISTER requests with a 403 error. The carrier side logs show the requests are being sent correctly with valid credentials. The error originates from the Genesys side.

We have verified:

  1. The carrierId matches the exact string from /api/v2/telephony/providers/edge/lines.
  2. IP allow-lists are correctly configured for the JMeter server IP.
  3. The SIP trunk is not marked as “Overloaded” in the admin console until after the 403s start.

JMeter Config:

  • Thread Group: 500 threads, ramp-up 10s, loop count 10.
  • Timer: Constant 100ms between requests.
  • SIP Sampler: Method REGISTER, Expires 600s.

The documentation mentions rate limiting on SIP signaling, but we do not see 429 errors. We see 403s. This suggests a credential or IP validation failure under load, which is confusing because the same credentials work fine at lower concurrency.

Has anyone seen this specific behavior with BYOC trunks in US1? Is there a hidden limit on concurrent REGISTER requests per trunk that isn’t documented? Or is this a bug in the edge registration handler?

Any insights would be appreciated. We are blocked on our capacity planning report.

2

Have you tried shifting the focus from raw SIP concurrency to workforce scheduling constraints? The 403 errors might not be a network issue but a capacity mismatch in your WFM configuration. When agents are scheduled into skills that route to this specific trunk, the platform validates availability before accepting the INVITE. If the schedule adherence logic blocks the assignment due to “Not Ready” states, the SIP stack rejects the connection with a 403.

  1. Review the shift parameters for agents assigned to the affected skill group.
  2. Ensure that “Available” status is explicitly mapped to allow trunk routing.
  3. Check if any auto-disposition rules are forcing agents into “Not Ready” during peak load.
  4. Adjust the shift swap settings to prevent mid-call status changes.

This approach stabilizes the registration by ensuring the WFM engine correctly predicts agent availability. It prevents the SIP trunk from receiving requests for agents who are technically offline in the scheduling system. Try aligning your JMeter test windows with published shift start times to see if the error rate drops.