At this point, the problem, is that the number blacklist applies to the entire system, not just one extension. So, although you may not want to receive any calls from the last number, others may feel differently. There would have to be an option that applied to each extension individually. Having this feature work with DID calls is one thing, but if a receptionist put the call through, then the call was "restricted", so that the call ended, or rang continuously, they may simply call back and complain to the receptionist, who may not know why the call failed.