Is this a Mitel Mivoice Office 250 (the old Intertel 5000) or a Mitel 5000 (Astra) platform?
Regardless, you would have to have the routing built to send it to your carrier... possibly requiring an external access code and the number, but your ARS or call routing needs to know how to route that call. We would need you to clarify the platform and how the call is routed, possibly the specific digits dialed as well.