Harassment

I have been getting calls at all hours from the call centre. I can't accept personal calls while at work so I reject the call each time. And within 30 seconds the call comes through again. Then it stops for a few hours until about 16:00. It went on,last night,until 21:00. If I do answer the call the person just hangs up. They have no reason to be contacting me,my account is up to date. I will take it further if this continues