Activating new router/modem

I'm back! A cable tech person came to the house and said the wiring is fine so I decided it was prob time to upgrade the modem/router. I bought the Arris Surfboard SBG8300 and got it set up. It is Cox approved and it says so right on the box too. When I click on the Activate your modem prompt it asks for my account info and pin, all goes well, then it asks for the Mac ID, I key it in. The HFC Mac ID is the same on the box, on the bottom of the modem and in the manual but when I key it in Cox tells me it doesn't support that modem. The website and the box both say they do. I noticed the Activate form inserts colons in the ID That prob has nothing to do with it. I tried calling tech support 3 times at diff numbers and got hangups on all 3.. one time said they were too busy, the other 2 just hung up. Then I remembered how helpful this group can be. I'm using my cell phone to send this message. Any ideas what I am doing wrong? (I used the SBG8300 setup doc From Cox' files to get it working)