The iPhone can enlarge text to make it more accessible for vision-impaired users,[146] and can accommodate hearing-impaired users with closed captioning and external TTY devices.[147] The iPhone 3GS also features white on black mode, VoiceOver (a screen reader), and zooming for impaired vision, and mono audio for limited hearing in one ear.[148] Apple regularly publishes Voluntary Product Accessibility Templates which explicitly state compliance with the U.S. regulation "Section 508".[149]