Voice simulations fail for some users but not others

Last updated: August 21, 2026

Your allow list can be correct and voice simulations can still fail, but only for some users. That specific pattern is almost always AI application isolation, not a firewall block. If you haven't applied the allow list yet, start with the network requirements article.

The Issue & Symptom

  • The simulation loads and the scenario appears, but the voice call never connects. It works for some users and fails for others with no clear pattern by office, region, or device.

    • Chat simulations are unaffected.

Why it happens

Secure web gateway and SASE platforms (ZScaler, Netskope, Palo Alto Prisma Access, Island, Menlo and others) can be configured to automatically isolate any application they classify as an AI tool that is not on their recognized list.

Isolation is not the same as blocking:

  • A block rejects the request outright. It produces a clear failure your team can find in logs or in the browser console.

  • Isolation silently reroutes the session through a remote browser. The request appears to succeed, so there is no obvious blocked-request error anywhere.

The Fix

Add vapi.ai to your AI isolation policy exceptions. This is a separate policy object from your general allow list, so adding it to one does not add it to the other. vapi.ai covers api.vapi.ai.

daily.co belongs on your standard allow list but is not usually classified as AI and rarely needs an isolation exception.

Confirming the policy is active

Quick check: have an affected user visit vapi.ai in a browser. If it loads inside an isolation frame or shows an isolation notice, the policy is active for that user.

If it loads normally but simulations still fail, that does not rule isolation out. vapi.ai and api.vapi.ai may be classified separately, and the simulation uses the latter. Try any other well-known AI site as a control: if that one isolates, the policy is live on that user and api.vapi.ai still needs an explicit exception.

Definitive check: on an affected machine, open Chrome DevTools, go to the Network tab, tick Preserve log, start a voice simulation, and filter for vapi. Compare against a machine where it works. On the isolated machine, the call setup request will not complete.Related

After adding the exception, re-test with a user who was reliably failing before, not one who was intermittently working.

Still failing? Contact support (support@solidroad.com) with the time of a failed attempt and the affected user's email, and we can confirm what the call attempt looked like from our side.