The object is missing from your outbound softphone layout.
Go to Setup->Customize->Call Centers->Softphone Layouts. In the picklist at the top choose Outbound. Add Case (or whatever object it is you're dialing from) to the list of available objects. You'll have to log out of Salesforce.com and log back in again for that change to take effect.
Please see the following Salesforce guides.
https://emea.salesforce.com/help/doc/user_ed.jsp?section=Personal_Setup&loc=help&target=cti_personalsettings.htm
https://emea.salesforce.com/help/doc/user_ed.jsp?section=cti_admin&loc=help&target=cti_admin_phonelayoutscreate.htm