One easy way to determinate if your call blocker is defective is by testing it using a different phone line.  If you do not have a second line at home, you may take it to your neighbors or relative and plug it in to confirm that it’s acting the same way.  If it continues to act the same way, then most likely you have a defective device. If it does not, then the device is okay, and most likely there is a compatibility issue with your phone line.