CALL-E webhooks carry no signature and no shared secret. Their issue #91 is open on exactly that, and it is not something a receiver can work around.
So anybody who learns this URL can post to it, and what they post looks identical to the real thing. Every event here is marked as a claim about a call rather than a reading of one, and every one of them wants a person, even the ones where all three readings are stated facts and they agree.
Treat the URL as a secret. It is the only thing between this inbox and the whole internet.
Events are kept for a day, fifty at a time, then they expire on their own.