This simple to use plug in amplifier will allow you to hear all your telephone calls clearly once more. Plugging discreetly into any standard telephone to give up to 100% increased volume so you can hear even the quietest callers. So simple to fit it features an on/off switch & volume control to suit each phone user & is ideal for the hard of hearing or phones located in noisy areas. Requires 1 x 9V battery (not supplied).