If you want to use Outlook 2007/2010 with TAPI you have 
to make sure to not use Office Communicator, otherwise Outlook 
tries to initiate the call via Office Communicator instead of TAPI.

In short, to use Outlook 2007/2010 with TAPI you have to (untested):
1. Add the following key to the registry:
    HKEY_LOCAL_MACHINE\Software\Microsoft\Office\Outlook\Call Integration

2. Add the dword-value TAPIOVERRIDE and set it to 1

3. Restart Outlook. 

If you are using 64bit version of Windows/Outlook try the following key instead (or create both keys):

    HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office\Outlook\Call Integration



For more details please refer to:
http://social.technet.microsoft.com/Forums/en-US/office2010/thread/b85aa37b-6360-4f9e-b478-19cf755a85e7
http://support.microsoft.com/kb/959625/
