I remember the times when I had an old Nokia phone as my main; there was some weird behaviour with Caller ID which was strange for me. When somebody with subscription to "Hide your number" service called me, instead of Caller ID "Hidden" was displayed except the cases when that person's number was in my contacts. However, there was something wrong with my land line because when I called my number from home phone it displayed "Unknown" even if the number was in my contacts. What is happening on the GSM/SS7/PSTN layer in those 2 cases?

2 Answers 2


Unknown: there is actually no caller id data given. Probably totally stripped by the sender, but can happen in other ways like between networks that have no conversion between the Caller ID standard they are using.

Hidden: the flag was set to say this is barred, the data is still sent all the way to the local telephone exchange of the receiver in most of the standards (there are several different caller Id standards): "When CNID is blocked at the caller's request, the number is actually transmitted through the entire telephone network, with the 'presentation withheld' flag set; the destination CO is expected to honor this flag, but sometimes does not"

  • Could you provide a reference to the standard and the page number?
    – assp1r1n3
    Jun 22, 2016 at 15:59
  • @assp1r1n3 As I said, there are several different standards used by different networks, some use different modulation, some send the data at a different time in the ring cycle, as a such the data is different too also not all these standards are freely accessible by the general public. Wikipedia does have a nice page on Caller ID: en.wikipedia.org/wiki/Caller_ID
    – ewanm89
    Jun 22, 2016 at 16:19
  • How come it possible that some of these standards are not available to the public? And which one(s)?
    – assp1r1n3
    Jun 22, 2016 at 16:51
  • I suspect that if you pay an organization somewhere, you could get access. Sort of the same situation on why you can't get some scientific papers without coughing up to the journal's owners
    – infixed
    Jun 22, 2016 at 19:45
  • @infixed Pretty much, also they can have you sign NDAs and the fees are at corporation pays organisation levels.
    – ewanm89
    Jun 22, 2016 at 19:54

Very good question!

Let me explain:

In every packet being sent to call centers there is a attribute of "hidden" this is a simple flag of true or false and so if the packet has this attribute set to "True" then the caller id will be hidden.

There is an attribute in the packet that holds the origin number that started the call. Since theses packets are editable this attribute is set to an empty number/erased attribute and so the call you are getting is actually from someone who stripped the origin number attribute in one of the two ways.


  • Some carriers may attach a random number(Example: 0000000) to the original caller id when attribute is empty/does not exists.
  • Some carrier may even drop a call with no attribute!
  • While a number with the hidden flag is still in the packet and is recoverable by the carrier a "unknown" cannot be recovered and so strong spammers prefer to strip the original number attribute.

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