This happens when CallerID signal of the telephone line is weak. Here are some possible solutions:
1) You may connect your telephone and the Call Blocker in parallel. Simply get a telephone splitter. Important: If you do this, then calls that are in the ‘Blacklist” may ring 2-3 times before they get disconnected by the Call Blocker.
2) Optionally, you may add a third-party CallerID device in series BEFORE the blocker so you can read the CALLERID of the incoming calls. Connections will look like this: Phone line > CallerID Device > Blocker > Telephone(s).