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.