With the growth of its popularity, more and more VoIP services are deployed in the Internet nowadays. Similarly to other Internet applications, VoIP users may desire to be unlinkable with their participated VoIP session records for privacy issues. In this paper, we explore the Items of Interests (IOIs) from anonymization aspects based on a simplified VoIP model and analyze the potential links between them. We address possible methods to break the links. Finally, we also discuss requirements for a VoIP Anonymization Service (VAS)