The latest Device Pack should be used. While you can search for the firs devicepack that the 7921 was introduced on your own, I recommend using the latest Device Pack.
ciscocm.4-2-3-DevPack-60
Link: http://tools.cisco.com/support/downloads/go/ImageList.x?relVer=4.2%283.60%29&mdfid=280264388&sftType=Unified+Communications+Manager%2FCallManager+Device+Packages&optPlat=&nodecount=2&edesignator=null&modelName=Cisco+Unified+CallManager+Version+4.2&tr...
Not sure the link will navigate for you. If not, go to Support > Software Download. Go to the voice software section. Navigate under IP Telephony>Call Control>Cisco Unified Communications Manager (CallManager)>Cisco CallManager Version 4.2
From here, you can pick "Unified Communications Managr/CallManager Device Packages".
I recommend the latest device package simply because I don't see the sense in bothering with scrounging through earlier releases. When you load the Device Package keep in mind that it will update firmware for ALL PHONES included in the package. If you do not want this new firmware pushed out to your non-7921 IP phones, you will need to take some additional measures. First, write down (or use a query to save) the current Device Defaults used by your cluster. When loading the Device Package on the Publisher, before you reboot modify the Device Defaults back to the firmware load IDs that were used prior to the Device Package install. You should only have to do this on the Publisher but it is a good idea to check the Device Defaults if/when you load the Device Package on the other servers in the cluster. Remember, when using a Device Package that contains new device definitations for your CM version, you must load it on all cluster nodes and you must reboot all nodes after loading the Device Package.
HTH.
Regards,
Bill
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