Resolution:
There is a MXM permission setting which allows or disallows calls to non-registered endpoints. It is on the Security/Non-Registered Devices tab at the MXM level of settings. You can also set the maximum number of such calls as well as the maximum bandwidth allowed for such calls.

Secondly, each user profile has an individual setting for allowing or disallowing calls to non-registered users. It is on the Bandwidth Control tab of each user profile.

Finally, the MXM property labeled "Don't Search In Other Zones" can be checked so that the MXM does not repeatedly try to find the target user in a neighbor zone, commonly resulting in an eventual call timeout. This setting can be found on the Gatekeeper/Zone Settings tab at the MXM level.