NCO Financial calls all the time!!

I made the mistake of moving to another state and not leaving a forwarding address as I thought I had contacted every important creditor I had. Needless to say, I missed one and that credit card bill, which I might add was only for $200 went to a debt collector.....NCO FINANCIAL!!
Now they are saying I owe HUNDREDS more and i have asked them to send me verification. I have never received that and the calls continue.

What can I do now???

Cindy