The correct method to set it up is IN-LINE as illustrated below. The main line cable from the main socket to your phone simply unplugs from your phone and plugs into the LINE 2 port of the Call Blocker (left side). The short black cable we supply ...
This is the CPR Call Blocker v.108 model. It has 200 pre-programmed numbers of worse known nuisance calls offenders stored in the call blocker. You can also block up to 1000 extra numbers at the touch of a button. CPR Call Blocker v108 User Guide