Harassment calls from any group, including the National Principal Group, are illegal and unacceptable. If you are receiving harassing calls from this or any other group, it is important to document the calls by writing down the date, time, and any relevant details about the call, such as the caller's phone number or any threatening or abusive language used. You should also contact your phone company and ask them to block the number, and you can report the calls to the police. If the calls continue, it is also important to contact a lawyer to understand your legal rights.
It's worth noting that National Principal Group is not a known organization and it could be possible that the calls are from a scammer pretending to be from that group.