Determinator is very easy to use. All you need to do is set the number of units from 4 to 7 (depending on your local telephone number system) and have it connected to the telephone circuit from the right.

Then you just plug it into your telephone set.

Incoming and outgoing calls are marked with icons. The numbers are shown as XXX-XX-XX.

With Determinator you can just look at your telephone cord to find out who is calling.