Recently I reconfigured my firewall (via Shorewall) to block all inbound traffic to port 5060, except for whitelisted IP addresses from my inbound DID providers. After setup, we ran tests and everything worked fine for all incoming calls across all providers.
Then a few hours later, calls from one of our providers started being blocked. All calls from our other providers continued coming through fine. Upon restarting our firewall service, the blocked calls from the single provider started coming through again.
Between our successful tests and the start of the issue, there were zero changes made to the server.
So why would my firewall suddenly start blocking inbound traffic from a whitelisted IP that it was previously letting through??