You need to create multiple Device Pools and Media Resource Group lists. The Device Pools use the corresponding MRGL. The first MRGL has the first subscriber services listed first and the other subscriber's services listed second. For the other MRGL, you define it vice versa.
You could, for example, differentiate between the HQ and the branch offices. Hence, you could point the branch office Device Pools to the 2nd MRGL (assuming that the total of your branch office phones is somewhat equal to the number of phones in the HQ.
Cisco recommend using a server load balancing appliance (or IOS license for Server Load Balancing on Catalyst 6500) to make IP phone services resilient.
This is not really realistic for smaller customers so I tend to configure the DNS servers used by the phones to use round robin to give out the addresses of all CCM servers in the cluster and set the phone service URL to use a name rather than IP address. Hopefully an example will make this clear.
ip dns server
ip domain round-robin
ip domain name example.net
ip host cucm.example.net 10.180.44.11 10.180.44.139 10.180.52.11
This enables DNS on a router and responds to queries for cucm.example.net (the name used in the services URL) with the following addresses in turn 10.180.44.11 10.180.44.139 10.180.52.11
Obviously if you have a server based DNS system available you can configure round robin on that instead of using IOS.
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