Enter a “match all” dial plan rule. Assuming that all of your current outbound dial plan rules have a domain entered, enter one additional rule with the lowest priority with nothing in the Domain field. This creates a “match all domains” rule.
Set it to the IP of your proxy server. For example if the “match all” rule is set to use 10.10.20.20 as the SIP Proxy to Use and the user dialed 12345, the call would go out to that proxy as email@example.com. How the proxy then handles the call is outside the scope of the Meeting Server.