Hi Ian,
This relationship is built via the Callmanager Group and Device Pool that the phones belong to.
You can specify where a phone registers by assigning it to a specific Device Pool this in turn will look at a Callmanager Group that lists a Primary, Secondary and Tertiary Callmanager server.
Here is some related info;
Cisco Unified CallManager groups and device pools represent logical groupings of devices that you can arrange in any way that you want. For ease of administration, make sure that all the devices in a group or pool share a common and easily identified characteristic, such as their physical location on the network.
You can also use Cisco Unified CallManager groups to establish redundancy (backup call processors) for the primary Cisco Unified CallManager in the group. A Cisco Unified CallManager group comprises an ordered list of up to three Cisco Unified CallManager servers. During normal operation, the first (primary) Cisco Unified CallManager in the group controls all device pools and devices that are assigned to that group. If the primary Cisco Unified CallManager in a group fails, control of the device pools and devices that are registered with the primary Cisco Unified CallManager transfers to the next Cisco Unified CallManager in the group list.
From this good doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00806b036a.html#wp1025126
Cisco Unified CallManager Groups
A Cisco Unified CallManager group comprises a prioritized list of up to three Cisco Unified CallManagers. The first Cisco Unified CallManager in the list serves as the primary Cisco Unified CallManager for that group, and the other members of the group serve as secondary (backup) Cisco Unified CallManagers.
Cisco Unified CallManager groups associate with devices through device pools. Each device belongs to a device pool, and each device pool specifies the Cisco Unified CallManager group for all of its devices.
Cisco Unified CallManager Groups:
Call processing load balancing You can configure device pools and Cisco Unified CallManager groups to distribute the control of devices across multiple Cisco Unified CallManagers.
Device Pools
Device pools provide a convenient way to define a set of common characteristics that can be assigned to devices.
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a008073ee5d.html#wp1059777
Hope this helps!
Rob
