Select: Setup > Objects and Fields > Object Manager > User> Page Layouts
Add the fields: ACD Extension Number
ACD Extension Number
This is a custom field where we can set the extension number for automatic call distribution.
In case, the field is not specified ACD for Salesforce will use the default Extension Number field for call distribution.
Note - The sole purpose of having a custom ACD Extension Number field is to have a separate field for call distribution in case the default extension number field is already used for some other purpose. Otherwise we can use the default Extension Number field for call distribution also.