Every operator offers a call-divert feature, allowing you to divert phone calls from one number to another. If for some reason, usually by mistake, someone has diverted their phone calls from their phone number to your phone number, you will receive all the calls directed to their number.
The first thing you need to find out is the phone number people are trying to dial. Then, you need to go to your operator and ask them to break the call-divert link.
Maybe someone calls you believing that you are someone else, because they have dialed the wrong number. Also, if you have recently purchased this phone number, it's possible that the other person tries to reach the person who owned the number before you.
Once you receive an unknown call, listen to what the person has to say and explain the situation to that person.
If you leave your phone number in online contact form, you risk being annoyed by unknown people. Leave it only when it's really necessary (i.e. when you see an "*" asterisk icon in the phone number field).