The majority of CTI APIs to date have been limited by their design to mainframe computers and minicomputers, and were difficult to adapt to use in personal computer platforms. Some early attempts to integrate telephony into personal computers required specially-designed PC hardware; an example is the "TeleCompaq" computer, that included an integrated handset and other special telephony hardware. In many cases, this specially-designed PC hardware was not expandable, insufficiently powerful for general computing needs, and introduced incompatibilities with some operating systems software and applications. This experience has shown that to be successful, integration must be based on standard PC hardware, and must use standard software and software development tools. TAPI builds on the Microsoft Windows foundation, using standard PC hardware and software, preserving users' investments, and allowing the flexibility to which PC users have become accustomed.