I have configured this:

1.- I created a Softkey template whit the sofkeys Callback in the call state On Hook y Ring Out.

    Device / Device Setting / Softkey template

2.- I don't find the option Softkey Template in the device pools. My options are:

    Device Pool Name* 
    Cisco Unified CallManager Group* 
    Calling Search Space for Auto-registration
    Reverted Call Focus Priority*

    Date/Time Group* 
    Region* 
    Media Resource Group List
    Location
    Network Locale
    SRST Reference*
    Connection Monitor Duration***
    Physical Location
    Device Mobility Group

    Device Mobility Calling Search Space
    AAR Calling Search Space
    AAR Group

3.- In the Phone configurarion from a phone I set the "Common Profile" as "MigratedCommonProfile10" and the Softkey Template set to "Standard User CallBack". The configuration of "MigratedCommonProfile10" is: Softkey Template :  "Standard User CallBack", User Locale: English".

4.- And the Call back parameters are the defaults:
Call Back Enabled Flag* : True
Call Back Notification Audio File Name* : Callback.raw
Notification Audio File Name*   CallBack.raw
Connection Proposal Type*   Connection Retention
Connection Response Type*   Default to Connection Retention
Call Back Request Protection T1 Timer (sec)*   10
Call Back Recall T3 Timer (sec)*   20
Call Back Calling Search Space   < None
No Path Reservation*   TrueFalse True
Set Private Numbering Plan for Call Back* True

I attach a picture whit the message: CallBack can not be actived on Press Exit to quit this screen