You can enable your call forwarding on the Verizon Website:
1. Go to the Call Forwarding page in My Verizon by clicking here.
2. Enter your deposit number in the Forward Mobile Number to field.
3. Select the following option in the Options section: Forward calls when my line is busy.
4. Click Set Up Call Forwarding.
Note: If there's more than one line on your account, select the line you want to set up Call Forwarding on from the Mobile Number dropdown.