If you DVd them. then you must have received a dunning letter from the CA.
If you responded to the address in their dunning letter, then your CMRRR DV response is adequate legal proof, and should shut down all collection activity. Keep copies!
NCO is not noted for its complance with the FDCPA.
It is not your burden to hunt them down.
you can DV as soon as you see the tradeline appear on your credit report - it is collection activity and constitutes contact with the customer.
send the DV letter to the Horsham, PA addresss, it's their corporate and legal office. include copy of the original one and the undeliverable notice.
you can also send CMRRR the copy of undeliverable notice to the CRA and tell them that it is a violation of the FCRA to publish the wrong or undeliverable address of a creditor.
you can use these GC as proof should you sue later.