Start by confirming that the forwarding works correctly when forwarding to some other extension number during the user's office hours.
You will need to carefully check settings for "Automatically switch status to Available during office hours", "Disable Outbound calls outside office hours", and the office hours that apply to the user's extension.
If the phone attempt to forward to an outside number fails, the logs should give you some hints as to the reason why.
Outside of this, your issue could be related to how the destination number for your "external" number is being processed by your outbound rules - make sure that the destination number is allowed through the outbound rules for a regular outbound call.