With the growth of its popularity, VoIP is increasingly popular 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 anonymisation aspects based on a simplified VoIP model and analyse the potential links between them. We address possible methods to break the links. Finally, we also discuss requirements for a VoIP anonymisation Service (VAS) in terms of functionality, performance and usability. Based on this, we discuss the fundamental design requirements for a VAS which we intend to subsequently implement.