I have several users reporting a code 4102503 when trying to make outgoing calls. They can received incoming calls and also voicemails show up in their Inbox. The laptops are definitely connected to the internet.
It sounds like there might be a firewall issue that is potentially blocking outgoing calls. The first thing would be to review the following document and insure that ports are open: Network Firewall and Proxy Settings
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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!