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CLIP No Screening a fairy tale?


VoIP protocols have made the manipulation of the “CallerID” easier for the regular IPBX user or admin. So users started to question the industry to relax “CallerID screening” to display the original calling party number on call forwarding (done by the PBX), or the number of the company’s Headquarters and so on. While “CallerID screening” has been around for a very long time and has been a default feature in most cases, only a few customers were able to benefit from a more relaxed screening.

With the rise of ITSPs, marketing departments invented “CLIP No Screening”, a kind of “send any CallerID and we’ll accept it”. I suppose most of you understand the value of the average email with a faked sender address, well imagine what kind of phone calls you’d get once everyone would offer “CLIP No Screening” to anybody who asked for it.

Luckily or sadly “CLIP No Screening” isn’t always implemented the same way. At large there 2 types of implementations:

#1 CLIP No Screening Implementation:

Forwards what the customer sends, don’t check, don’t sanitize, actually do nothing to the “CallerID” provided by the customer (except maybe apply some dial plan manipulation to convert the number to E.164 format).

#2 CLIP No Screening Implementation:

This actually works pretty the same as #1 except that the “CallerID” provided by the customer’s equipment is transported along the real Calling Line ID of the customer’s SIP trunk. In order to distinguish both numbers, these 2 numbers are usually called “User Provided Number (UPN)” and “Network Provided Number (NPN)” respectively.
The UPN is transported in the “From:” header within SIP and as an “Additional Calling Party Number” in a “Generic Number” information element within SS7(1). The NPN is transported in the “P-Asserted-Identity:” Header within SIP and as the “Calling Line Identification” within SS7.

The second implementation allows to show a different “CallerID” to the called party but still allows to determinate the source of the call in case of need.

Unfortunately the second implementation doesn’t work for each destination. Some network operators don’t support to display the additional number, others are blocking the feature. It is open to discussion if blocking the 2nd implementation drives more users and service providers to use the first implementation.


(1) Signalling System No. 7: telephony signalling protocols used in most public switched telephone networks.

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