Have you checked your Trunk Group Priority ordered number list under 14-05? If they are still set to the default settings and include Trunks in the ordered number list that are not installed on the system, you will have issues (such as busy signals/disconnects) when you attempt to do call forwarding off-premise. This would particularly an issue if higher priority numbers such as 1 or 2 are assigned to non-existent trunks.