As per the federal laws in the fdcpa, collection agencies are not allowed to call you frequently in a particular day. Section 806(5) prohibits contacting the consumer by telephone "repeatedly or continuously with intent to annoy, abuse, or harass any person at the called number."
Keep a log file of all the calls made to you by the agency. Record the dates and time so that you are in the process of filing a legal case against them. Send a cease and desist letter and make the first legal step to stop their calls. If they persist, you will have more charges of filing a case against them. Send your letter through certified mail with return receipt requested.