STARPLUS Triad XTS User Manual page 111

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The B/Z ringer switch is located on the bottom of the
telephone near the line jacks. This switch should be set to the
Z mode for business/hospitality applications and to the B
mode for residential applications.
The MSG light and the Line In Use (LIU) LEDs will not
operate when set in the B ringer mode.
For single line telephones that are not equipped with a
volume control slider switch or to increase the volume
greater than that allowed by the slider switch, use the
following steps.
1. Dial [638].
2. Dial a valid number (0-9) to select a volume level.
3. Press HOLD.
Volume Control