To set up call forwarding on a Windows 8 phone, first ensure that 4G is not enabled (Call Forwarding is only supported on 3G or lower). Next, set up call forwarding via one of the two methods listed below:
Click on the Phone app, then click the 3 dots on the bottom right hand side (…). Next click Settings and set up call forwarding here.
If call forwarding is not available in the above option, try this method instead:
Click Settings -> Network+ (Note that this is not the same as the Network+wireless button) -> Call settings (Set button) -> Activate call waiting (slide the button to the right) and enter a number to forward calls to based on the conditions listed on the phone. Then after you’ve entered the number you want to forward calls to, make sure to tick the box to forward calls to that number, under those conditions
Call forwarding should now be activated. If neither option is available to enable call forwarding, maybe its not supported by your phone or service provider.