If you tell a creditor the number they called is your work number and tell they are not allowed to call that number then they may not continue to call that number. I would encourage you to do this in writing. The actions of creditors' are governed by the Fair Debt Collection Practices Act. Check out the link below to learn more about the rights the Fair Debt Collection Practices Act gives you.
As a strategy, before you tell a creditor to "buzz off," be sure you milk them for all the information you need from them, such as the name of the collector, their address, their phone number, their fax number, the amount they claim you owe, the account number. I urge you to use any dramatic skills you have (without lying) to get them to give you all this information with the thought that if you were convinced that the debt was legitimate, you would immediately pay it.
Then once you have "sucker punched" the collector, tell them you will investigate their claims and not to call again. Follow up by mail & by fax. And if you want to investigate the debt, start by looking at your credit reports - I have posted a link to be able to order the free federal annual credit report.