They keep calling and they have changed the area code consistantly over time.  Every so many days they started with 313 and now are up to 972.  And like the rest when you answer it is complete silence.

Forward it to their domain registrar   570-708-8780 thats what I'm doing. Let someone that has to deal with them suffer. Maybe people will not want to do business with them if you annoy the people they work with.

NCO Collections they work with GE Money/Sam's Club Credit.  They call from several different numbers and area codes. like 951-289-4371 or 443-837-2763.  They will call 10 or more times a day and never leave a message, and when you do talk to them if you don't have the amount of money they want  you to pay, the will not work with you.

I just never answer the phone unless i know who it is

Here's a link to several methods to stopping unwanted collection calls; you must have the account # that NCO is referencing though.
http://credit.about.com/lr/stopping_collection_calls/47632/4/