Add a Scan in message when an items call number has been changed through Scan in items> Change item information, when the call number field has been left blank.

This could be an optional message enabled through "Scan In Messages Configuration" where you could select either none, pop up message or feedback message.

The workflow I am referring to is here
https://knowledge.exlibrisgroup.com/Alma/Product_Documentation/010Alma_Online_Help_(English)/030Fulfillment/020Resource_Requests/040Scanning_Items

It would happen when the call number is empty, and it is being moved to an existing holding with a different call number.