Hi Antonio,
This registration is determined using the relationship between Device Pools and Callmanager Groups. By design, 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. Perhaps these phone models are in different Device Pools/CCM Groups. Here is some related info;
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
