The evolution of mobile phone functionality in recent years is quite remarkable. One example is the approach to music, with FM tone generators used for playing ringtones appearing in mobile phones around 1999. Since then, mobile phone tone generators have continued to evolve, reaching sound quality equivalent to that of PC-based softsynths. Using the numeric keypad for text messaging, mobile phone users—particularly younger users—are able to communicate easily and with great agility. As using the mobile phone keypad as a musical instrument spreads, it is not unimaginable that users could become as virtuosic as on any other musical instrument. However, if each mobile phone maker independently implements the way the numeric keypad is used to play music, this could mean that a user would have to re-learn a new interface for each mobile phone. This document provides a suggestion for standardizing the user interface specification for using the mobile phone as a musical instrument. Rather than be restricted to mobile phone devices’ numeric keypads, guidelines for also using the QWERTY keypads of mobile phones and PCs as a musical instrument has been appended since version 1.0.3 of this document as Section 3.