Designed for use by disabled passengers, especially visually impaired, Series 56 meets European Technical Specifications of Interoperability for Persons with Reduced Mobility (TSI-PRM) standards. Design features large pushbutton within bright yellow triangular bezel, which is visible in all light levels. Also, SOS is printed in ink and Braille text. Letters are 15 mm high, and Braille is raised more than 0.5 mm high.