Calling line identification presentation (CLIP) allows the calling party's number and subscriber name to be displayed to the called party. When a call is placed, the originating phone switch sends the caller ID information, which the receiving telephone company looks up in a database to identify the subscriber name. However, caller ID can be blocked, trapped, or spoofed. Blocking prevents display of the caller ID by dialing a code, while trapping still sends the blocked number through the network where it can be identified. Spoofing involves generating fake caller ID information that does not match the actual calling number.

1. CALLING LINE
IDENTIFICATION
PRESENTATION
(CLIP)

2. The Way ahead…
• Calling-line identification
• Operation
• Uses it
• Blocking
• Trapping
• Spoofing
• Disabling
• Enabling

3. Calling-line identification
• The calling number and the billing (or subscriber) name where available.
• When an originating phone switch sends out a phone number as caller ID,
the telephone company receiving the call is responsible for looking up the
name of the subscriber in a database.
• Cell phone companies must support interconnecting trunks to a significant
number of Wireline and PSTN access carriers.
– PSTN :- The combination of the interconnected networks and the single numbering plan make it
possible for any phone in the world to dial any other phone.

4. Operation
• Caller ID information is sent to the called party by the telephone switch as
an analog data stream using modulation.
• Database are used to Identify the caller information.
• E.g. For 800 numbers, they may report a string such as TOLLFREE
NUMBER if the name is not available in a database.

5. Uses it
• Telemarketing
– Some telemarketers have used caller ID itself for marketing
– E.g. an 800 number and the text string "FREE MONEY" or "FREE PLANE
TICKETS" as the name to be displayed on the caller ID.
• ISPs
– Some ISPs providing dial-up access require the customer to use CNID to
prevent abuse of the account by unauthorized callers.
• Mobile providers

6. Blocking
• Blocking is the common term for preventing the display of a calling
number.
• By dialing a special code before making a call to block the calling
number.
• E.g. code is *67, "PRIVATE" or "WITHHELD

7. Trapping
• if you block your number, it can still be trapped as it is still sent
through the telephone company network.
• When the local number is called by a customer who is blocking their
caller ID, the local number is forwarded to a toll-free number, where
the number of the calling party is passed on to and the owner of the
local number can receive the information.
• When a customer rejects an incoming blocked call it is routed to
TrapCall, who in turn traps the number and reroutes the call back to
the cellphone user with the Caller ID displayed giving the customer
the option to answer the call or reject it once more to send it back to
TrapCall where the second call will be sent to voicemail.

8. Spoofing
• Caller ID is a simple string of data that can be read and generated very
simply by small computer programs and even some inexpensive
devices.
• VoIP providers use Caller ID replacement to save local plan minutes.
• E.g. Skype service provider.

9. Disabling
• On GSM mobile networks, callers may dial #31# before the number
they wish to call to disable it.

10. Enabling
• On GSM mobile networks, callers may dial *31# before the number
they wish to call to enable caller ID.