Accounts Receivables Management 866-679-4046

I have received numerous phone calls from this company, both on my cell phone and at my work number. Most if their messages state for me or my legal representative to call 866-679-4046 within an hour regarding an important business matter. In looking up this phone # it appears it is tied to Accounts Receivables Management for debt collection. I have no idea whom they may be trying to collect for and their phone calls are sounding threatening. I have not received any communication in writing from these people so I have no idea what this business matter is for. Is this phone # tied to Accounts Receivable Mgmt. of Florida OR the one in New Jersey.
Do I wait until I receive some written letter from them BEFORE I send a cease and desist and debt validation letter?