Two things come to mind.
I started receiving spam calls from a man that was propositioning a business deal. Somehow he had both mine and my husband's cell numbers. We were able to block his calls through the phone provider.
The second scenario is a scam warned about by the federal government. It involves calls that come in to a cell phone that can appear to be by local numbers but are actually coming in from overseas. When they call and you answer or call the number back from caller ID, your account gets charged per minute for long-distance. So the advisory warning is to not answer and not call back.
I don't know if either situation applies in your case, but I think the best thing to do would be to report it to your phone company and ignore it.