It's not a wise idea to block "Unavailable" numbers, although, you could probably get away with it 95% of the time.
Here's how things work. Anonymous Call Rejection will block "Private" callers only. It can do this because a special flag is sent along with that call's caller ID information that explicitly tells the recipients phones not to display that caller ID info. Phones that don't play by the rules can unmask it anyway. If TWC's phone systems see an incoming call with that flag set, it doesn't forward it to your phones. That's how it rejects it.
For "Unavailable" calls, there's nothing that gets sent. No caller ID info, no flag, nothing. TWC can't (really they shouldn't) act without that information. As a result, you cannot guarantee whether it's a legitimate or fake call. The information could simply have been lost in transit. Many doctors offices will call with "Unavailable" numbers so that they remain unlisted from hypochondriac patients, but at the same time, allows them to get around the Anonymous Call Rejection. That's the 5% reason why you shouldn't block those calls. Although it's not in the spirit of the protocol to do that, it's not illegal either. At least, it's not enforced.
I believe NOMOROBO will only act on calls that have caller ID info. As I mentioned above, it's to guarantee the legitimacy of a block/rejection.
After some research using a resource I won't mention shows that the caller(s) (who shows up twice as "UNAVAILABLE" and once as "PRIVATE") are actually spoofing my home phone number! Looks like the latest countermeasure being utilized by some illegal telemarketers. It appears to be a local "Home Improvement Contractor" that calls a few minutes prior to these calls and gets blocked by NOMOROBO.
Having worked on a Nortel DMS 500 switch in the past, It would be very simple to allow the option for customers to select the ability to reject calls that show up as "Unavailable" on their CID display. This option should be made available to customers to use if they wish. I know that it most likely will not be offered as it is used by many doctors calling from their private phones, along with Emergency Alert systems and political campaigners. The big problem now is that many of the telemarketers and robocallers are beginning to use it big time (which is against the law and a Federal Offense) to get around both carrier-offered specific number blocking, anonymous call reject, and the NOMOROBO service.
In typical TWC fashion when I attempted to set
Anonymous call rejection, after about 1 minute of waiting for the transaction to complete I got an ERROR message.
Virtually everytime I attempt to alter my phone settings TWC errors out.
Without exception Time Warner Services is the slowest web site on the Internet. They must use DSL on their end.
OK, now a few days later, I once again tried to set Anonymous call blocking but now as soon as I select Privacy in my phone settings the web site waits for at least a min then gives me a terminal error.
Every day TWC pushes me to look at antennas, Aereo, Netflix.... with their horrendous service.
How can a broadband service provider have the slowest and most erratic web site on the net?
One thing that would be real helpful is to allow spoofed phone numbers that begin with "1" after the area code to be blocked. Lately I have been getting loads of calls from the following numbers and the TW selective call blocking system will not allow me to input these numbers. NOMOROBO blocks them all, but I still get one ring and it usually occurs during my afternoon "siesta"
Unfortunately neigher I nor TWC has been able to get my modem settings to allow NOMOROBO which would greatly help the situation.
I am now seriously looking at Ooma which maintains their own community generated scammer list. It is also much cheaper after the initial purchase price and will easily be paid for in 5 months.
I loved Nomorobo as it caught 3-5 calls per day, sometime 3 an hour. HOWEVER I just had to give it up because it causes callers from some TimeWarner landlines to hear 1 ring and then hear the line go dead - while on my end, I hear regular ringing. If the caller hangs on through the dead air, I will answer (if I am here) or eventually they get voice mail. But most callers won't do that.
It took 7 calls to TimeWarner (most with Tier 3 phone techs) and one tech visit to discern the problem was the call-forwarding that Nomorobo uses. Even though I told every tech I talked to that I had recently signed up for Nomorobo, all said they had never seen the problem before -- until the last tech who had experienced the same problem with Nomorobo with her own phone on a different carrier. She said they have tickets open on this problem with TWC/Nomorobo.
Soooo if you are having odd problems with apparently dropped calls, think first of Nomorobo. It might save you some valuable time -- and a LOT of frustration. I miss Nomorobo, but it's unacceptable to have my favorite callers (including my 90+ year old mom) not able to get through!
The CID of UNAVAILABLE shows normally when anyone disables their caller ID when making a call, so none of the carriers and NOMOROBO would want to allow blocking of this in fear that you may miss an important call. The problem of late is that telemarketers have figured this out and are now starting to spoof UNAVAILABLE. The way that I get around this was to download VoiceZone Enhanced from this site: https://code.google.com/p/vz-enhanced/
and put the first caller who calls with a CID of UNAVAILABLE into the IGNORE list. Another way would be to purchase a blocker device called a Teleblocker from Ebay and it will allow you to set a configuration to block this CID.