CLID on forwarded call to PSTN

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Apr 26th, 2007
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I have a location that is having an issue with forwarded calls CLID. When I call forward all my phone to an external number (i.e. Cell Phone) I lose the calling number display. I have verified that my gateway is passing the correct calling party number to telco, but telco is overwriting with the BTN. Regular outbound calls pass the correct CLID. Telco claims that this is a security feature and is not allowed on purpose. They only allow calling number display for numbers assigned to the local PRI. Why shouldn't I be able to forward my phone to my cell and see the calling number of who is actually calling me? This works at many other sites.

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Rob Huffman Thu, 04/26/2007 - 15:01
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Hi Jeff,

I know that this is probably not the answer you want to hear but, this Telco is handling this properly. The sites where this is working are not being handled correctly by the Telco. The ability to pass non-assigned numbers through the PRI opens up an area of possible abuse that could be very problematic. It seems rather benign on the surface but what if?? This also means that you can probably assign the local Police name/number or others on an IP Phone and spoof calls. As far as the Forwarding part goes, your Telco is actually being quite kind to just mask these with the BTN. Many providers will fail the call coming from your Gateways if they do not have a number associated with your DID Blocks. Here is a doc that explains this;

Failure to Forward Call Offsite - Issue with Calling Device Number

From this doc;

Here is a recent thread where Paolo,Vlaho and I were talking about this exact issue;

Hope this helps!


paolo bevilacqua Thu, 04/26/2007 - 17:05
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Actually there is a mechanism in Q.931 (ETSI variant) that allows the the recipient of a forwarded call to see the originator caller number, without compromising security.

It's called call deflection. It works like this, A calls B, B on per-call basis tells telco that the call has to be forwarded to C. C sees A's number in the call.

So far the good news, now the not so good;

- Is not widely adopted by telcos. Only few North Europeans ones have it (the usual suspects).

- cisco does not supports it.

Perhaps we will have it in the future, given the right amount of asking and kicking.


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