Note: This blog post is from 2007. Some content may be outdated--though not necessarily. Same with links and subsequent comments from myself or others. Corrections are welcome, in the comments. And I may revise the content as necessary.
Jared Rypka-Hauer today
wrote of a hassle he's experiencing with Comcast Digital Voice (their phone over cable service), and he asked if others had thoughts. I may well have an answer for him, or perhaps for others even with other similar phone-over-cable service providers. My wife and I were also suffering with it recently, until a rep found and fixed our problem--a hardware problem. Almost classic, from a geek perspective. :-) Perhaps you too may have it.
Did you previously have another phone over cable service before, like Comcast's "Digital Phone"?
The key question is whether you by any chance had the predecessor to Digital Voice, called Digital Phone (or perhaps any other phone-over-cable service that was like it rather than like Digital Voice, as I'll explain). If so, there may be a problem of something that was left behind in a switch to DV from DP, which could be conflicting with the new DV service. Removing that seems to have solved the problem for us.
Apologies to those who don't like wordy posts, but I want to explain things clearly for the benefit of others in the same boat.
DV service was bad for us--until this was fixed
My wife and I have been Digital Voice customers for about a year, and especially recently we grew VERY frustrated with it for some of the same reasons Jared describes, such as dropped calls, but also failed faxes, problems with the internet service, and even worse, we would occasionally lose all the phone service.
When we lost all phone service, if we connected a single phone to the digital modem (MTA) (inside the house, which feeds the phone line into a jack to serve the whole house), there was indeed signal. So they asserted it "must be something in our house", and they'd tell us to unplug all the phones in the house and wait a half hour, then reconnect. It would always work, but what a hassle! Kind of like a software provider pulling the old "reboot your computer" answer to a tech support question.
Naturally, my wife was fuming and investigating alternatives, when a couple weeks ago we had a rep come out to have a look (what to us was one last look) at things. What he found may well be the clue for Jared or others who had Digital Phone (or anything like it) before Digital Voice. And note that if we simply switched cable service providers, the problem may well have propagated into the new service (since the root cause wasn't the service to the house after all, but it also wasn't a problem inside the house).
What was Digital Phone? And what may be left behind from it?
Comcast's first step into cable-based phone service was Digital Phone. What's the difference? Well, those with the newer Digital Voice (DV) may have noticed that with it, the cable comes into the digital modem (or "MTA") which is inside your house, and out of that comes the phone signal via a normal phone wire that that's fed from the MTA into a normal wall phone jack somewhere nearby inside the house, and that then feeds the rest of the house its signal. There's no more need for any phone line to come into the house, so any outside wire like at the plain old telephone system (POTS) network interface device (or "NID") is disconnected. (picture)
But you may (as we) for a while have used Comcast Digital Phone (which came out before Comcast Digital Voice). And it's important to understand the difference, for this problem. DP worked in a way that was closer to the plain old telephone system (also referred to as POTS). The comcast coax cable from the street went first into a box that was on the OUTSIDE of the house (very much like the one used by the POTS, but a different one: picture). And out of that DP box came both the cable (which went on into the house like it had before installing DP) and also out of it came a phone line, which then was connected to the old phone system box/NID a few feet away, specifically to the phone jacks inside it that then fed phone signal into the house just as it always had in the POTS. (here's a picture)
When they switched us to Digital Voice last year, naturally that phone line into the house (through the POTS NID) was no longer needed, so whoever did the DV install of course removed the phone line going from the DP box into the NID to feed the house, and they set up the MTA inside the house, and so on as needed for the newer DV.
But here's the kicker and what the guy found 2 weeks ago: the DV installer had not only left the old DP box (outside, which he should have removed), but he also left the cable (coax) running through that outside DP box (updated info) and inside of it, it was going through a unit intended to take the DP phone signal out to feed it to the POTS phone box. Then the cable line went out of that box and on into the house. Most important, the rep pointed out that that unit inside the DP box was powered (they had tapped into the power line when they installed that DP box a couple years ago).
Power to the old DP box was interfering with the cable signal
Well, the problem was that the power to that box (and the unit inside through which the cable signal was passing) was directly interfering with the cable signal going out (which of course feeds the TV, phone/DV, and internet service). That would explain a lot of the problems we were having!
So he removed the old DP box (and its power line) updated: opened the old DP box and removed the powered unit inside of it, leaving the box but also disconnecting the power to it. More important he connected the cable inside the unit at that point (what went from the street, and what went into the house) so it was now uninterrupted. The signal is effectively "pure" going into the house, where it then goes to the MTA (the digital modem), and that (as for the last year) feeds the phone line, and the internet service and cable TV that are from there fed into the rest of the house.
So far all has been much better.
Your next steps
If you're not sure if this may be an issue for you (maybe someone had it setup even before you moved into a house in recent years), go out and check if there's a box outside your house on a wall near your phone box that looks like the old phone box. Or find where you cable comes into the house and see if it goes through some other box first, since they may have installed the DP box (or its equivalent) elsewhere outside your house and run a long line to the old phone box. Since that old phone line will likely have been removed, you may not see that.
If you have one of these boxes, and the cable goes into it and then out of it and on into your house, you may well have this problem. Another clue will be if you do (or don't) see a thick power cable feeding into the box. Since it's powered, you won't be able to open the DP box (well, you may find it has a "consumer" side that you can open, just like the old phone system NID, for you to test the phone line outside the house, but that won't show you the cable line to see if it's going through a powered unit as I described.)
But if you have something like this, call Comcast (or your provider, if in a situation like this) to have someone come out and confirm and if so remove it (which should be free, I'd think, especially if they offered the old service to you).
You may then enjoy DV, internet, and tv a whole lot more.
Could be a broader problem
Again, the problem may be generic to other cable systems (RoadRunner, Adephia, Time Warner, Cox, Charter) if they also have offered two generations of phone-over-cable service (or your house may have had it, even if from another provider, before you moved in).
Since setting up things like DV is mostly an "all inside" thing now, some installers may never even think to look at this potential problem. All they may do is unplug the old phone line going into the house and come in and setup the MTA and say, "enjoy". I wonder how many people could be suffering this problem and not know it.
Hope that helps someone.
Update
After writing the entry, I went out to take pictures of the units (which I've added as links above, keeping them rather hi-res to make them more legible). I realized I was mistaken when I said he had removed the old DP box. He had not. But what he did was remove the powered unit inside it and disconnected the power to it (he said) and left the box. I've corrected that above. The problem with this, though, for your diagnosis is that you have no way of knowing--if you do see that second box, the DP one--whether the cable line inside it is still going through the powered unit that should have been removed. You really have no choice, it seems, but to ask Comcast (or your provider) to come out and ocnfirm this, if you are having problems that seem otherwise unresolvable. Sorry for the confusion, but still, I hope it all helps someone.
Today I'd like to offer still more, from yet another vendor: 
Have you ever wanted to see the Java source code for the proxy/stub that's created when you invoke a web service from Coldfusion? Well, here's a hidden gem in CF8 (one of dozens I discuss in my "hidden gems in cf8" talk) that does just this. 
