This is an IP Office security issue. We don't have all the answers as to why a customer would receive a 503 Service Unavailable error when trying to register virtual handsets for Multimedia. The partner is primarily responsible and can raise a ticket with Avaya to move forward if no resolution is found.
Extension Number and Password are the Same
Historically this is not a problem but there is a security setting that, if enabled, prevents any user with matching credentials (ext and pw) to register. This is the case with the below setting applied (found in Security Settings►General).
Best to just not have the password be the same as the extension.
TLS protocol is activated and/or media security is too high
*these steps may be temporary to verify if they are the cause for the 503 error. If so, yet the customer need to re-activate these security measures, they'll need to open a ticket with Avaya to determine how to allow the virtual handset(s) to register.
Disable TLS in the System►LAN1►VoIP
Media security is found in System►VoIP Security. Try at a lower or disabled security level for testing.