Most likely a debt scamming operation.  The word "complaint" is meant to scare you into thinking there's a law case filed against you.  If there were a law case being contemplated against you, you'd have heard about it from the attorney for the other side.  There'd be a paper trail.  You can check this out yourself by simply going down to your lcal main civil court and have the clerks check your name against the defendants civil index.  Most court clerks are familiar with these sorts of scams and will be glad to help you out.

If they won't give you a debt validation letter by US mail (E-mail no good), simply block their calls and ignore them.