Question:
Why does the offline validation rule, that is using Is_Parent_Call_vod__c, not immediately firing when conditions are met?
Answer:
Is_Parent_Call_vod__c is a formula field. The formula results are evaluated at sync time.
Consider using the Parent_Call_vod__c within the Offline validation rule instead of Is_Parent_Call_vod__c.
Related Documentation:
CRM Help Documentation: Offline Validation Rules