ITU-T Rec. E.161 – ARRANGEMENT OF FIGURES, LETTERS AND SYMBOLS ON TELEPHONES AND OTHER DEVICES THAT CAN BE USED FOR GAINING A
1 Use of figures and letters in telephone numbers
2 Rotary dials (see Figure 1/E.161)
3 Pushbuttons or keys
    3.1 10 pushbuttons
    3.2 12 pushbuttons
    3.3 Dual mode and engraving
    3.4 Design of symbols
    3.5 Use of colours
    3.6 Position of figures, letters and symbols on push-button sets
4 Additional pushbuttons for use on telephones
    4.1 General
    4.2 Specific recommendations
Reference