1. Create a MRG for each resource at each site. HQ conf, HQ MOH, HQ Xcode, same for BR1. Do not add any unused resources to any group. The devices with MRGL assigned can access only the resources in the groups and does not have access to any unused resources. Total 6 MRG’s required.
2. Create a MRGL for each type of device, one for phones one for gateways. Total 2 MRGL’s
3. if one site requires Mulitcast and another requires unicast. Then uncheck “Allow multicast” under the appropriate MRG. The audio source should be multicast enabled.
4. It seems gateways cannot access MRGL from the Device pool. So you have to apply MRGL at the gateway level. All other devices (phones etc) are able to access MRGL from the Device pool if you have not set a MRGL under the device.
CCIE Voice notes 
“The devices with MRGL assigned can access only the resources in the groups and does not have access to any unused resources”
This is not entirely true, any devices not assigned to a particular MRG resides in the “default MRG” which is accessible by all devices and is searched last in the MRGL order, It’s easier to think of this in the same fashion as an DN/RP/TP residing in the null partition which is the last PT in every CSS. Just to make sure that a particular device is NOT used I typically add it to an unused-MRG and leave it unassigned to any MRGL (this is helpful for things like SW CFB etc.)
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a00806e8c25.html#wp1094998
*clip* SRND
•The system defines a default media resource group that is not visible in the user interface and that all resources are members of when they are created. Consumers of media resources use resources first from any media resource group (MRG) or media resource group list (MRGL) that their configuration specifies. If the required resource is not available, the default MRG is searched for the resource. For simple deployments, the default MRG alone may be used.
•When using MRGs to control access to resources, it is necessary to move the resources out of the default MRG by explicitly configuring them in some other group. If the desired effect is for resources to be available only as a last resort for all calls, then the resources may remain in the default group. Also, if no control over resources is necessary, they may remain in the default group.
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