It is advisable to be careful about regional accents. Volume control is needed. The same knob or slide used for volume control can however be used for switching off the speech. Good contrast is required between text or figures and buttons. It is recommended to have a combination of a first letter in upper case, with following letters in lower case.
Text is required to be as large as possible.
Abbreviations can be used as long as they are understandable and explained in the instructions. It is recommended that the pressure required to operate the keys be the same for all keys, even for larger ones. If a 00 key is needed, it should be same size as the other digit keys and placed below the 2 key. It is advisable not to have a mark on the 1 key. The on key should be easy to find and identify, no matter its shape or size.