Spent a few minutes looking around and there are some OK pieces of hardware that would work but all seem to have quirks that would be troublesome for us.
Not the least of which being their complexity and setup time (although some look to be pretty simply if you don't need/want all the features). I am not sure why you are "hung up" [pun intended] on the call waiting issue, though. If you are already on the phone, then it isn't that much of an interruption (if you have call waiting caller ID). Worst case, just ignore it; or even disable call waiting (*70) (yep, I don't have call waiting because it costs money I don't want to pay... it is rare I am on the phone all that long). In which case, the Tel-Lynx looks like it might be a great fit for you.
The best solution, strictly from a logical level, would be to have something that screened all calls through a huge data base of 'bottom- feeders' and let everyone / anyone else through. The best service does seem to be nomorobo but as I said, COX does not support it.
In my case, that wouldn't be the best, because those still allow crap through like the city "notices", political calls, etc (from what I understand). Personally, I would never want or accept ANY robo-recording call, marketing call, political call, sales call, "notice" call, etc. Plus, although it has honeypots, blacklists only work if people participate and mark calls as spam.
We have reached a state of OK in that all ringers are off, so a caller either has to talk to my answering machine, or when I get the chance and look at missed calls, I can call back whomever I choose. That will have to do for the moment.
Kinda selfish, though.... if the other party does the same thing, then the two of you will never communicate. Plus it is still very inconvenient- you don't know from across the room if it is important or not until you stop, go over, and do whatever to get it playing back the message; and then delete it. That is, if you even remember. I get recorded voicemail from spammers, usually a partial of the recording, sometimes even a human (although that is rare). So voicemail/answering machines do not help much with many types of phone spam. In fact, I think it makes it even more annoying- most are calls from when I am not even home.
But just to reiterate, I am really cranky about having to deal with any of this: my phones are devices I pay for, both the hardware and usage fees. And it really rubs my fur the wrong way that others are using my own purchased devices to torture me. Not my world, I just live here.
Your crankiness about spam calls is probably only matched by mine. And I don't even get that many spam calls. If I were at your level, I would have changed my number or ripped the phone(s) out of the wall if I couldn't find something to help.