Probe Hold

What the SYNTHETIC CALLER's own leg does: nothing, for long enough.

⛔THE PROBE NEEDS A CALLER WHO STAYS ON THE LINE. The first live run of synthetic_call used Plivo's public answer.xml, which ends immediately -- the call connected, was recorded, and lasted five seconds, so it proved the carrier could reach us and nothing else. Holding the line is what lets the greeting play, the options play, the tree fall through to Rooster, and a transcript exist to judge.

⛔PUBLIC AND UNSIGNED, DELIBERATELY, AND IT IS THE ONE ROUTE HERE THAT MAY BE. Every other voice webhook is signed because it acts on a real call -- it books callbacks, reads accounts, opens tickets. This returns four fixed words of XML, reads nothing, writes nothing, and names nobody. A signature here would gate the instrument on the very thing the instrument exists to check.