There is something called call manager group , check the groups you created or you want to create , when you enter the config of the group , you can select the call managers and then reorder them as you want.
To let this be affective , on the device pool config , select the group you want so when the phone get's the device pool , he will get this config along with the region (for codecs) , location , SRST reference (if applicalble) and the most important is the media resources.
Amer
