I'm no Mitel expert and learning on the fly. If you have any screenshots or things for me to check, let me know the path. Thanks!
I am not around a system at the moment...
Go to Maintenance Commands, enter 'DGT TRACE 55041111' and it will output how/where the call is routed. Because your digits dialed sends it to a list, there are multiple routes so you need to go ARS Route Lists and see what routes are being used by List 3, and look at each route and how the digits are modified on that route, if that route has the correct COR applied to it, etc.
To be honest, know that I think of it, your issue is probably a conflicting dialing plan... I would start in maintenance commands and, because the error occurs at 55, enter 'LOCATE NUMBER 55' and see the result, there is probably a feature access code or speed dial that needs to be changed because it is conflicting with the ARS.