There is a maintenance access code. Typically 555. From the Maintenance port (Slot 1, cct 1) you can dial the maintenance code + trunk acess code. Intructions are in the manual (if you have one)
There might be a plug in or terminals on the front of the cabinet to access the Maint port. There is on a SX200 but I'm not sure of the 100 Cabinet.
I havent touched an Mitel analog cabinet like yours in over 10 years so my info is necessarily vague. (like my memory)
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Occam's Razor - All things being equal, the simplest solution is the right one.