No need to change the default value if you only want to return active users, just update the 'Reference Specification' tab and the Reference Qual Condition to ACTIVE is TRUE.
Then if you don't auto-populate the caller field with the current user and need to lookup users, it will only return active-true users, is that your intention?
By default that incidentGetCaller() will autopopulate the caller field if the user logging the incident is not an itil user, otherwise it presumes they are logging a call on behalf of someone so will use the reference qualifier to only show you active users.
