I can’t seem to figure out why the Genesys Cloud SIP trunk returns a 503 Service Unavailable error when the JMeter script ramps up to 500 concurrent calls. The environment is set up with a dedicated SIP trunk for load testing, and the capacity planning document suggests this volume should be supported. However, the error starts occurring consistently after 300 concurrent sessions are established.
The error log shows:
SIP/2.0 503 Service Unavailable
Call-ID: <[email protected]>
Here is what has been done so far:
- Increased the JMeter thread group ramp-up time from 10 seconds to 60 seconds to reduce the initial burst pressure on the SIP proxy.
- Verified the SIP trunk configuration in Genesys Cloud Admin, ensuring the maximum concurrent call limit is set to 1000 and that no authentication failures are logged.
The WebSocket connections for the softphone remain stable, but the SIP signaling fails. Is there a hidden rate limit on the SIP proxy for new call setups per second? The current setup uses JMeter 5.6.2 with the SIP plugin. Any insights on how to debug the SIP proxy capacity would be helpful.
The documentation actually says SIP trunk capacity is governed by concurrent session limits, not just registered agents, so you likely hit the 300-session hard cap before the 500-user load completes. Check the trunk’s max_concurrent_sessions parameter in Genesys Cloud admin and increase it, or verify if your BYOC edge is throttling the ingest rate.
You might want to check at the interaction between your BYOC edge configuration and the Genesys Cloud platform’s rate limiting mechanisms before simply increasing the session count. While the suggestion above correctly identifies the concurrent session cap, merely raising the max_concurrent_sessions limit in the Genesys Cloud admin console can lead to more severe issues if the underlying network infrastructure or the BYOC provider cannot handle the increased signaling load.
In our experience building integrations that scale across multiple organizations, hitting a 503 Service Unavailable during load testing often indicates that the SIP trunk is being throttled by the ingress firewall or the session border controller (SBC) due to rapid connection attempts, rather than a hard limit in Genesys Cloud itself. The platform API enforces strict rate limits on registration and invite messages to protect the service mesh. If you push 500 concurrent invites in a short window, the platform may reject new connections to maintain stability for other tenants.
Consider implementing a staggered ramp-up in your JMeter script to mimic realistic user behavior. Additionally, verify the max_concurrent_sessions parameter in your BYOC provider’s dashboard, as this often has a lower default threshold than Genesys Cloud. You can check the current trunk status and limits via the Platform API:
GET /api/v2/architect/schedules/siptrunks/{sipTrunkId}
Ensure the response indicates sufficient capacity. For a detailed breakdown of SIP trunk scaling best practices and rate limit considerations, refer to this support article: SIP Trunk Load Testing and Rate Limiting Guidelines. Ignoring these limits can result in temporary trunk suspension, which would halt all traffic during your test.