It may depend on location and service type
In the UK by default (for PRI) if your extn number matches your DDI number with same number of incoming digits then you do not need to program the CPN substitution form
Normally if the extns do not match the DDI then CPN substitution is required to get the CLI displayed as a DDI number, however if the CPN substitution is not programmed and the extn number does not match a number in the DDI range then the default (main number) is displayed on outgoing calls, this is controlled by the carrier as the default CPN, but can be selected/changed in the default CPN form if another number belonging to the DDI range is to be dislayed.
For DASS2 the CPN substitution forms will not work and the carrier controls the outgoing CLI and number. Again if the extn number matches a number in the DDI range then it will be displayed only if the DASS2/OLI/TLI is enabled on both the trunk and extn COS.
If the DASS2/OLI/TLI is off then only the first number of the DDI range is displayed.
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