This probe is locked

Not a wrong passcode: there is no passcode set, so nothing opens it. The probe can start a real voice call, and a call spends money, so with no door configured it stays shut rather than standing open.

To open it, set PROBE_PASSCODE on the Vercel project to any word you like, then redeploy. This screen becomes the passcode form and that word is what it asks for.

Vercel’s own protection would have been the tidier answer and is not available here: both Vercel Authentication and password protection refuse this plan for production deployments.