Okay, now I see the purpose. What I do is set up an Answer DN on both telephones. DN 221, for instance, would have a key to answer 222. And DN 222 would have a key to answer 221. Simple.
As far as routing to off-premise number, you would need a T1 or PRI because the MICS will not let you forward a DN to a particular number if it is using "9" as the access code. But if it sees "9" as part of the dial string (meaning it does not instantly switch over to a trunk before you dial) then yes it can be done. You would have to permanently forward a DN to the outbound number. We use this at our office to forward our extensions to our cellphones when we're not in the office.
As far as how a Definity system goes, I do not know because I do not work on nor service a Definity PBX. Definity is a true PBX, so it probably uses something like phantom DN's.
As far as purpose, the MICS is to Avaya's Partner system (both are considered KSU's) as the Option 11/61/81 is to the Definity (both are considered PBX's).
John Panzer