You will need to set your additional allowed extension as “200-299,300-399”. Since your main number is “623xxxxxxx” and you need 3 digit extension length, Default Allowed Extension will be “000-099”. So the system will not allow you to choose 0 for operator.
**FOLLOWUP QUESTION FROM PARTNER**
What is the significance of the “main number”? It looks like we can translate inbound numbers, so can we just arbitrarily assign a main number where the 3rd digit from the end is not 0 and then use 0 for operator?
The “Main Number” is equivalent to the “Front Desk / Operator’s” Number. When you create a user, by default the “External Caller ID” for that user will be set to the “Main Number”. The reason is, if the user does not have a DID, when a call is made from his/her phone the caller number will be sent as the main number. This way, the PSTN side can call