The number of rings depends on 2 factors:
1) If you connect your phone(s) in series (as shown above), your phone(s) should not ring on blacklisted numbers. If you connect it in parallel, your phone(s) may ring 1-2 times before the call blocker disconnect the blacklisted calls.
2) Also, most telephone providers send the Caller id information ahead of the ring signals, giving the call blocker enough time to check the blacklist and ‘suppress’ the first ring signals. However, there are a few telephone providers that send the caller id info at the same time they send the ring signal, allowing no time for the call blocker to suppress the rings. In these cases, you may ring 1-2 rings, even if you have it connected in series.