This symptom is strongly indicative of the telco not understanding NI-2 Protocols. (Please confirm that you are using NI-2)
With NI-2, the Mitel uses the protocol to send a call flag to indicate the type of call and the digits required to identify the call.
For LD, Operator, and Overseas calls, the call type flag replaces the steering digits for these areas. (1, 0, and 01 or 011) and the system only sends the remaining digits.
When the Telco understands the protocol, everything works.
When the Telco does not understand, it only acts on the digits received and the call fails.
One fix is the one you have discovered whereby you add an additional 1 for LD. Adding an additional 011 would likewise work for overseas.
Fortunately, the Mitel has accounted for Telcos that don't understand the protocol and have a protocal option for "Unknown Call Type". When this option is selected, all of the digits for all call types are sent to the Telco with an unknown call type flag (again the flag is probably ignored by Telco). What matters is that the Telco receives all of the digits.
Form = ISDN Protocol
Option = Enable Unknown TON/NP
Description:
True or False. Check the box to change to True. This option is only available when NI-2 protocol variant is selected. When checked, this option modifies the NI-2 protocol to send "unknown/unknown" as the Type of Number (TON)/Numbering Plan (NP) in the Called Party Number Identifier of an outgoing call.
Default Value = False
**********************************************
What's most important is that you realise ... There is no spoon.