The manual sent with my handset mentions that a PIN # needs to be entered before registering with the base unit. This was not the case. I guess and assume since its default PIN is 0000 it doesnt need to be entered unless you go into the settings and change the number (which is a good idea for added security).So after pressing "reg" on the handset, I than pressed the buttion on the base unit. Two lights lit-up for a bit than went off. I than released the button on the base unit and the handset was than registered.Other than that, this is a great additional handset. Each handset you register even indicates by a number on the screen which one it is. So they are easy to keep track of.Extra handset allows you to transfer a call to a specific handset, make 3way calls, make intercom calls between 2 handsets and you can be on Skype and a landline call at the same time!