This happens sometimes because the user very close to their local Central Office Switch (CO) so the CO picks up the numbers as the user dials them, and completes the calls before the device can cut off (e.g. disconnect) the calls.
To prevent this, if you are trying to block a 10 digit number, that the calls are still going thru ... keep reducing the number of digits until it works. For example if you are trying to block calls to 305-558-5577,
First try blocking 9 digits ... 305-558-557
If it does not work, then then try blocking 8 digits ... 305-558-55
If it does not work, then then try blocking 7 digits ... 305-558-5
If it does not work, then then try blocking 6 digits ... 305-558
6 digits (area code + exchange) should be enough to stop the local CO from dialing the number before the blocker disconnects the call.
This is even more common when blocking to 3 digits e.g. 411 calls. In those cases, try entering 2 digits into the memory.
For example if you are trying to block calls to 411,
First try blocking 2 digits ... 41
If it does not work, then then try blocking 1 digit ... 4
Note: The only issue with this is that it will block all calls that start with 4