The primary extension radio button tells the system what the primary extension is on the IP phone. For example, if you have a 7960, you may have 6 extensions, some of which may be shared, but 1 is your specific extension and associated with your voicemail box.
If you have a 7960 or a 7940 phone you can select the line you want to unconditionally forward, push cfwdall softkey, and enter the extension you want it to unconditionally forward to. If you want to remove the forward, select the line again and push cfwdall again and it will remove the forward. You can also do this from the call manager by going into the phone and selecting the line and filling in the call forward all box.
Hope this helps...
Amber