You may have a number of public short codes for public services.
Perhaps these are being handled as internal extensions rather than PSTN calls.
Perhaps you are dialling bespoke numbers that do not translate as expected.
Microsoft expects everything to be dialled in the e164 format
The service expects anything 6 digits or less to be an internal extension
So What can be Done?
Take a look at Microsoft DialPlan Management
Create and Manage Dial Plans