You are missing a “/”, but otherwise I don’t understand the problem. _X. allows you to forward the dialled digits without enumerating every possibility.
are you trying to dial from asterisk through the Mitel and back into asterisk again?
if so… in your Mitel Class of service make sure “public network to public network connection allowed” is enabled
also make sure “allow DPNSS access” I think its called is enabled
these would be enabled in the mitel class of service for ANY and ALL parties that will touch the call… this includes the TRUNKS AND STATIONS…
-Christopher