I am sure your system is different from the US's. Typically, when nuisance calls happen and no one is on the other end when the phone is answered, it is a sign that some piece of scum is checking to see if the line is active. According to the many threads on this matter, the cretins who sell our numbers get a premium for selling only confirmed numbers.
The US has had a "do not call list" for many years that is supposed to be respected. Unfortunately, the measure was passed and allowed only a civil penalty, which means the offended party has to identify the caller, serve them with a lawsuit, and win in court. Damages are very limited, e.g. $500 to $1000 for all that effort. The hardest part is to identify the caller who often is not in the US, but uses spoofed numbers. I have even seen my own home number on caller ID when receiving one of these nuisance calls. Such calls are not considered harassment in the legal sense, so law enforcement can't help.
My solution is not to answer the phone unless I recognize the party, and whenever I am at home, I turn off call answering. Even when I am not at home, I usually leave it off unless expecting a call.
John