They can call you any day of the week, during 'reasonable' hours (which I believe to be 8:30 a.m. to 8:00 p.m. - someone please correct me if I'm wrong), and they can call your home, work or cell phone - until you instruct them not to. (Best way to avoid that in the future is not to give any potential creditor your work number - only the home number - that way they won't be calling you at work, unless they manage to find your work number somewhere.)
If you ask them to cease calling you at work, the best way to handle it is to tell them that you are not allowed to receive personal calls at work, and tell them to call you at home. If you don't want them to contact you by phone at all, send them a letter indicating that they may only communicate with you in writing.