Automatic Number Identification and anonymous calls

Home » Asterisk Users » Automatic Number Identification and anonymous calls
Asterisk Users 6 Comments


This morning I called an analog line from a cell phone.

For the first call, I made a standard call and could both read my CID
and ANI in my Asterisk console.
For the second call, I choosed to hide my ID and, to my surprise, I
read that my CID and ANI were both empty.

I always thought that ANI (Automatic Number Identification) could not
directly be set or changed by end users.
In this experiment, it seems that if an end user calls anonymously,
the ANI is also hidden to the receiving party.

Is this standard practice ?
Does it depends on the telco networks the call is passing through ?
Thoughts ?


6 thoughts on - Automatic Number Identification and anonymous calls

  • I never work with analog lines, but I do believe that anything less than a
    PRI would have the blocking done at the telco end before you get it.

  • Mostly correct; ANI is only delivered over ISDN, SS7 and *some* E&M
    trunk circuits. Analog circuits have no ability to transport ANI.

  • 2012/2/8, Kevin P. Fleming :

    Thanks for telling ANI feature can’t come along analog lines.

    I did another test towards an ISDN BRI line and got the same results :
    ANI and CID are either both displayed or both hidden at the same time.

    Comments are welcome.

  • You need to talk to your provider to see if your circuit is provisioned
    to send you ANI at all; if it’s not sent by the provider, Asterisk just
    duplicates the CLID into the ANI channel variable. Most end-customer
    (retail) ISDN circuits are not configured to send ANI unless the
    customer asks for it.

  • Hello,

    I know about the german phone system that the sense of an anonymous call
    is, that the called party has no way to get the caller’s number in any way.
    The last switch honours the “anonymous” bit and removes the phone
    numbers before sending the call to the called party.

    In EURO-ISDN you have a feature called “CLIRO” which means “Calling Line
    Identification Restriction Override” and is only availiable to emergency
    services or special governmental services.
    With this line feature the last switch ignores the anonymous bit and you
    can see both caller id and ANI.

    The US phone system may work in another way, but if I call another
    person and want to hide my number I would appreciate if my phone number
    is not sent to the called party in any field.
    So this issue sounds to me more like feature than a bug 😉

    Greetings from Wuppertal
    Max Grobecker

    Am 09.02.2012 11:12, schrieb Olivier:

  • 2012/2/9, Maximilian Grobecker :

    Yes, I agree but what I rate as a bug is “asterisk copying CID data
    into ANI field”.
    The reasoning is asterisk should don’t modify, unless explicitely
    configured to do so, whatever comes from the public network side.

    For instance, it seems analog lines cannot support ANI. So when
    receiving a call through an analog line, this ANI field should be
    simply empty.
    It seems that today default is to copy CID into ANI field. This is
    misleading as you may think this ANI is somehow “more trustable” than
    CID alone.

    I agree that if a caller requires anonymity, then, except for very
    special cases (emergency services, for instance), ANI should also be
    hidden by the telco.