Howto:How does CLIP no screening work

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CLIP (calling line identification presentation) no screening is a provisioning mode which allows you to send any number as calling line to the ISDN network. Here is how it works


Applies To

This information applies to

  • Any ISDN trunk line

More Information

Problem Details

CLIP (calling line identification presentation) no screening is a provisioning mode for your ISDN trunk line which can be configured by your network provider upon request.

It allows you to send calling line identification information with outgoing calls which will not be verified (screened) by your network provider.

This feature is useful in various scenarios. E.g.:

  • you have remote locations in a PBX and your locations are calling out through your central ISDN trunk. In this case, outgoing calls originating in a location can send the locations local trunk number as calling line identification. However, when the provider screens the number provided, it will consider it invalid (since the number is not assigned to the trunk line the call is made on) and will replace it by the trunks original number. With clip no screening, the locations number will be sent untouched with the outgoing call
  • You forward or transfer external calls to extern (e.g. by setting a call forwarding to your cellular for your local extension). In this case, the original caller id will be sent along with the outgoing call and again be rejected and replaced by the switch. With clip no screening the forwarded call will reach the destination (your cellular in the example) with the correct calling line id

Technically, when clip no screening is activated, in fact 2 calling line ids are sent. First the one provided by the user, which is then attributed as user provided, not screened. Second the number determined by the provider as your trunk number, attributed as network provided, screened. The majority of terminals receiving such calls will just ignore the second number and "believe" (and therefore display) the user provided number. However, some devices (such as routers with verify calling line id feature) may be able to handle both numbers and trigger on the network provided number.

System Requirements

Please note that the feature must be provisioned by the provider supplying the network termination, not the provider handling your calls. So if you have pre-selection enabled, then your calls will be handled by an alternative carrier, however, the clip no screening feature must be enabled by the provider that implements the last mile.


In principle, there is no special configuration required in the PBX or gateway to use CLIP no screening. The gateway will simply send the normal calling line id as before and the carrier will convey that as user provided number and add an additional, network provided number to the call data. However, when this feature is enabled, calling party numbers sent as unknown type of number will usually be ignored by the switch.

So for the feature to work, you will need to configure proper Calling party number out mappings in the ISDN interface specs to change the type of number to subscriber, national or international. Surprisingly, aside from that, the numbers are blindly transferred by the carrier to the call destination. That is, when for example a subscriber number is sent, then it will be presented at the call destination as a subscriber number. So to the called party the call will appear as a local call from within its own local area. Similarly, a call sent with a national calling party number will appear as a national number when you are calling a foreign destination.

This is clearly undesirable, so the calling party number must be set to the appropriate type depending on the call destination. Also, the calling party number received from the PBX must be inspected carefully, since for calls originating locally, the local trunk number and area code must be prepended (since the PBX will send the extension number only). However, for forwarded calls (which will appear with the original calling line id at the isdn interface), the calling party number must obviously be left as it is. Fortunately, today incoming calls always appear with a national number as calling line id (that is, even when called locally, you will see the callers area code). So you can safely assume that calls not originating from PBX extension always will start with 0.

When configuring your mappings, make sure to obey these rules:

Called party national
Calling party number
Starts with 0 No change to apply
else Prepend local area code and trunk number
Called Party international
Calling party number
Starts with 00 No change to apply
Starts with 0 Prepend local country code
Else Prepend local area code and trunk number

The modification of the calling number can be done in the routing table (as it depends on the called number).

The modification of the type of number must be done in the interface mappings

Known Problems

In Germany, when clip no screening is enabled, then the second calling line id is always set to the lowest, single-digit extension number defined for your trunk. That is, usually the extension -0 is sent along with your trunk number. This has the slight disadvantage that when turning on the feature, call destinations which in fact care for the network provided, screened number will see a different calling line id after (as they previously received the correct extension of the calling terminal and will now see the -0).

Also, the user provided number may not be conveyed to all destinations. In Germany, such numbers are not sent to the following destinations: old national ISDN (1TR6) interfaces, analogue trunk bundles, simple ISDN basic rate interfaces. Some carrier interconnects may remove the user provided number when passing a call. For example, we have seen this for calls from German Telecom to Italy. In this case, since the defaulted network provided number is left only, the destination user will not see the callers extension, since the defaulted number always carries the "main" extension (-0 in that case).

You should consider using partial rerouting.

Related Articles

Support:Wizard Config fails for international calls

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