Call Forward All External Domestic Number

Hello. I am having an issue setting up call forwarding on our UC520. Call forwarding works to local external numbers but does not work for long-distance external numbers... The number we are trying to forward to can be called directly from any phone without a problem. Only when call-forwarding is set, we get a message that "we are sorry, your call did not go through"
Any assistance would be greatly appreciated.
Thanks!!