Large ITSPs (Internet Telephony Service Providers) often need highly available server infrastructure. Achieving high reliability is a crucial need in competition with traditional telecom operators. Building such a system requires a reliable database used in an efficient manner. At the beginning we will briefly overview the design and anatomy of a scalable VoIP (Voice over IP) server system based on SIP (Session Initiation Protocol) and MySQL database. Such systems typically consist of several components using the same back-end database. Utilization of the database depends on the type and volume of network traffic generated by end users. We will examine data collected from real-world servers to see the type and amount of database traffic for large (tens of thousands of subscribers) setups. Finally, we will take a look at common problems experienced while running large scale SIP signaling servers with MySQL as the database, with special focus on high-availability issues.