The second person sounds like the first person I talked to. He was very abrupt and wanted to know details of where I purchased it, etc. He made it sound like I was trying to do something illegal. The next rep I talked to mentioned there was a note on my file the next time I called, explaining the SN was a cablecard and was not supported.
I've called about 5 times since then ( I bought a dct6200 and needed that many calls to get it working - so I asked them at the end of each call about activating the SN) with no luck. They were however, much more polite than the first guy. They would pass me on to their colleague to enter the Serial number, but that second person always seemed to recognize it as a cablecard and tell me they could not add it to the system.