This happened to a customer Asterisk server and it somehow found the ID of the registration account to the upstream SIP server and was railing connection attempts (it filled up the console and there were literally thousands per second). Basically this caused all incoming and outgoing calls to fail.
It was a temporary fix but the solution was to block that specific IP, it's hard to stop it 100% because the customer needs the default SIP port.
asterisk, dos, peer, register, chan_sip, register_verify, configured, dynamicthis, server, registration, upstream, sip, railing, attempts, console, incoming, outgoing, ip, default,