Harassment for debt that is not even mine

I have been receiving sms's and calls from MBD attorneys, they are looking for my sister and I gave them her mobile cell number. But now they are harassing me at work calling me everyday for my sister. I do not live with my sister and don't understand why they are calling me for debts. I don't have any contract with them so they must stop calling me otherwise they leave me no choice but to take legal steps against them. I want all the calling to stop.,they must stop calling me as I am not the person they are looking for.