A debt collector can call your place of employment unless the debt collector knows or has reason to know that your employer does not allow you to receive such calls. If your employer prohibits such contact, then you should inform the debt collector immediately. However, it seems as though you're dealing with an original creditor? If so, then the original creditor is not subject to the FDCPA, but may be subject to your state's counterpart. I would check into that. For a short discussion of what the difference is between a creditor and debt collector, you can find it here: http://www.ericsapirlaw.com/faq.
If the company that is calling you is in fact a debt collector, and if you do what them to stop calling you period, a simple cease and desist letter (send certified mail with return receipt requested) should do the trick. You can find sample letters all over the internet that suffice or contact an attorney.
So, to answer your question... It is not illegal for a debt collector to call you at work, BUT from what you're saying, it appears as though the debt collector has called you from a spoofed number. If that is the case, then contact an attorney.
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