In the Vivox v5 SDKs, StatusCode 20503 is returned when joining a channel if the domain portion of the channel URI is different than the realm. This type of error is usually a typo.
For example, if the Vivox domain supporting an application's voice chat integration is vdx5.vivox.com, a request using vx_req_sessiongropup_add_session with a URI parameter similar to sip:confctl-g-issuer-value.testchannel@wronghost.vivox.com returns a VxXmppErrorServiceUnavailble for the status_code in the response.