I was expecting a call yesterday and it didn't arrive... and when I called the other party today, they said that the call was blocked, because my line has anonymous call blocking turned on.
Now anonymous call blocking is a nice feature if you want to avoid telemarketers who block their caller ID, which is approximately none of them, since they can simply insert fake caller ID instead and get around such blocking! But if you ever need to call the "doctor on call" for an off-hours medical emergency, for instance, the doctor on call will usually have call blocking (because they're calling you back from home). And some government agencies, as in my recent case, also block caller ID. So you don't always want this "feature" on. And I never ordered it or turned it on.
But somehow my Comcast Digital Voice line was set to block anonymous calls. So if you subscriber to CDV, you may want to log in to your account and check your settings -- you can turn it on and off yourself from their web site.
They're getting sloppier and sloppier as they get bigger and bigger.