We have received this feedback from a few customers using some models of wireless handsets e.g. Vtech model 5153C.  This happens due to a number of reasons:

1) Interference with other telecom equipment
Solution:  Try using a longer cord so you can allow at least 2 feet separation between the Call BLocker and the rest of your Telecom equipment (including modems, wi-fi routers,  phone systems, etc). 

2) Compatibility issues among some wireless handsets
We are investigating this issue.  In the meantime, we recommend that you use a different handset (preferably a cheap corded handset will do!).

IMPORTANT: Please keep in mind that there is no need to pre-program or preset your call blocker when you first get it set up.  This is truly a plug-and-play device that you can setup (in series or parallel), and start blocking unwanted calls by simply pressing the ‘BLACKLIST’ button.