Comcast keeps calling me and leaving messages (I don't answer) saying that my modem "is not capable of receiving the full range of their speeds". This has been going on for months. I upgraded to a Motorola SB6121, a DOCSIS 3.0 modem, about 6 months ago. Everything has been working fine, speeds are around 30 Mb/s download. So I finally decided to call Comcast and... pretty much just see what they had to say. I figured at best I could get them to stop calling me.
After almost 3 minutes of menu surfing, I finally was on my way to speak to a human. The person I talked to verified my identity and had a look at my account. She asked me if I had 2 modems. (Wondering why this matters): "I only have one connected..." Apparently the reason why she asked is that the other modem was shown as a Comcast rental. I have ALWAYS owned all of my modems. She put me through to the "Internet department" (haha) and I spoke to someone else, who asked me most of the same questions. After I explained that I own all of my modems, she said she would need to create a ticket, and someone would do "research", and call me back.