Based on your 400 errors after initial calls with high concurrency, this likely indicates you're exceeding Vapi's concurrency limits. Contact Vapi support immediately at support@vapi.ai with:

Subject: Urgent - Concurrency Limit Issue
Details:
- Account ID: [your ID]
- Error messages: [exact 400 error text]
- Timestamp of failures
- Number of concurrent calls attempted

According to [Vapi documentation](https://docs.vapi.ai/billing/billing-limits), even with purchased concurrency, there may be additional system limits affecting your setup. Sources: - [Vapi Billing Limits](https://docs.vapi.ai/billing/billing-limits) - [Discord Channel](https://discord.com/channels/1211482211119796234/1334142600772456508)

solved

checking if this is resolved/solved for you?

No

can you please share screenshot of the error and/or loom recording? we will then investigate the issue