I just went through this with Comcast, and I feel like I was lied to every step of the way. Long story short it turns out you CANNOT purchase a cable modem compatible with Comcast's gigabit offering, because they ALSO require that the modem supports their Internet Voice service, and ONLY Comcast Modems support that. Grimey fuckers.
The long story is: I called into Tech Support because I wanted to upgrade to their Super Triple Play, because getting virtually EVERY channel and upgraded gigabit internet turned out to cost $10 LESS than I was paying for my stripped-down service offering. Though, this was only the case if I purchased my own modem and eliminated the modem fee - which is great, because renting equipment is insane. So the comcast tech told me I should be good to go, but I needed to upgrade to a DOCSIS 3.1 compatible modem (my current modem was only DOCSIS 3.0 compatible). Their recommendation was that I go ahead and use their modem for a month to make installation easier, and buy a modem later to save time.
I went on ahead and scheduled the upgrade, and quickly ordered a DOCSIS 3.1 compatible modem (newest Arris Surfboard they make) - Amazon Prime got it to me in 2 days, boo-yah. I called Comcast Tech Support again to make sure my modem was good, and that I could use my modem during the setup instead of theirs, and they said it was fine. Again they said their recommendation was to use their modem during setup and replace theirs with mine later, but that the plan was still solid.
Tech guy shows up, calls in my MAC address, everything initially seemed to go as scheduled. However, I wasn't getting gigabit speeds, but he said that should work itself out in about 30 minutes, once the account is "closed" and everything finalizes (I also couldn't see HBO, which also meant the account hadn't finalized). He closes the account, calls in to make sure everything is good, and leaves.
8 hours later, still cannot access HBO and still don't have gigabit speeds. I call back, and the tech tells me that my account hasn't been closed yet. The service tech still needs to close it out, and I should have the tech come back tomorrow to close it out. I argue that I won't be home, I have a goddamn job and all, and I demand to know what hasn't been closed out about the account that requires a service tech. They told me my voice service hasn't been set up, and it's required in order to close the account. They highly recommend that I have the tech come back, use their modem to set up the voice service since the modem I bought doesn't support voice, and then replace the modem with a different compatible modem later.
So I got into an argument about my modem, that I was told my modem was compatible with gigabit internet, and he kept telling me I needed to refer to the online info about supported third-party modems. He kept recommending I have the support tech come back, install their modem, and find a third-party replacement later. After about 10 minutes on the call, the tech starts acting like he can't hear me any more. Bullshit. Then he says if I can hear him to call back later. And then he hangs up.
So I call back, and explain my situation and the jackwad I had just talked to, and the new tech dude repeated the "I recommend you use our modem to set up the service, and replace with a new compatible modem." So at that point I said there isn't a modem compatible with their Voice service, and it was not within my budget to rent a modem from them. The dude didn't say a word more to me, but forwarded my call to someone else.
The new person answered, and I asked if I needed to repeat my situation. She said nope, she knew what the problem was. She verified that gigabit internet from Comcast only works with THEIR modem, if I wanted to use any other modem I had to back down to the fastest non-gigabit offering. And then she recommended several ways I could even further reduce my bill by asking me specific questions about how I use my DVR and other services, and even recommended a Roku with their new app instead of renting a cable box. I was so happy to talk with someone who wasn't a fucking iddiot that I didn't complain about the jackwads I had talked to, or the crap I had been through at that point. And within minutes my service was enabled and everything worked. And on top of that, she waived the service tech/setup fee and lowered my bill another $10/mo.
But goddamn it, Comcast, it would be nice if you didn't keep trying to trick me into using your modem and locking me into a service when you KNEW I couldn't buy a compatible modem. Jackwads, all of them.