When creating a call forward you will be prompted for the forwarding destination. This is the phone number that calls to any number associated with the forward will be diverted to. The forwarding destination can be changed at any point after creation. The phone number should be entered in international format, for example the UK number 02035888000 would become 442035888000.
Once the call forward has been created it will appear in a list of forwarding destinations. You can then manage the forward using the buttons to the right of each forward.
