This is an extremely effective unit with 4 high pitch and 4 low pitch settings.  It is very simple to adjust the setting to make a woman sound like a man, or a man sound like a woman.  Alternatively you may want to play a trick on a friend and make yourself sound like a Dalek.  The unit is very simple to fit and can easily be moved from one phone to another.  Powered by 3 x ‘AA’ (1.5V) batteries (supplied).

This does not work on cordless phones.