The DID on FXO trunk is the incoming called number, which typically will be assigned to the main number of that company or the Autoattendant number, that callers can use to reach other extensions.
It is a combination of extension length and the prefix and total length is same as length of main number that is configured on System - Dial Plan page. In your case, looks like the total length of digits configured on main number is 6 digits.
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