@JRudey
Good day. The Zoom error codes can be broken down into segments. The last three numbers in the error code are standard internet error codes. So, your error above would break down to 503 - Service Unavailable. If you have one-way audio, then like @igorn said, that is typically a firewall issue and when you take that with the 503 error code, I would guess it's either firewall rules or SIP ALG rules on your firewall. Hope this is helpful!
--Bill