Concurrency, not daily volume, is the binding constraint. Ten thousand dial attempts a day, at an assumed 67.5 seconds of line time per attempt across an 11-hour window, needs about 17 lines on average and roughly 34 at an assumed 2× peak. Vapi includes 10 concurrent call slots by default and Retell AI 20 on pay-as-you-go (both checked 2 September 2026).
What this covers
- Converting a daily target into a concurrency requirement
- Carrier and platform limits, as published by the vendors
- Number reputation and answer rates
- Latency, provider rate limits and retry storms
- Compliance error rates multiply
This is a technical summary. The full guide — with the tables and worked examples — is on our site: *Scaling AI Calling: Concurrency Limits at 10,000 Calls*.
Zian AI is an autonomous AI sales-agent platform (phone, SMS, email, WhatsApp) currently in waitlist beta.
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