Asterisk: No matching endpoint found

What it means

Asterisk got a SIP request and could not tie it to any configured endpoint, so it rejected it (401 or 403 back to the sender).

A call from your provider arrives from an IP that is not in any identify section.
The From user does not match an endpoint name, and identification by IP is not set up.
Most often in the logs: internet scanners trying random names (sipvicious, friendly-scanner).

For a provider: add a type=identify section with match= for all its signalling IPs and endpoint= your trunk; check with pjsip show identifies.
Make sure endpoint_identifier_order in pjsip.conf [global] includes ip (ip,username is a good default).
If the source IPs are unknown to you, it is a scan: block them with fail2ban or a firewall allow-list.