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transferring external call via AA


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When transferring external pstn call (number A) via Auto Attendant be pressing "1" (number B ) to another external pstn number (number C), only the number B is shown on the screen C. But i need to show the number A.


PSTN <-SS7-> call agent <-SIP-> SNOM


Can all advise?

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I assume this is a call from an associated cell phone. Generally, PBX sends the ANI of the extension(number B ) or the domain/trunk ANI when making outbound calls. But depending on the trunk settings you can probably see the original caller id too. The INVITE sent out from the PBX should show what numbers are sent in the headers.

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These calls are made from any external pstn numbers.

Outgoing INVITE contained Message headers From: number_A, To: number_C, PPI (or PAI, depending on the trunk settings): number_B.

But the IAM contaiend only Calling party number (number_B ) and Called party number (number_C)... where redirection?

I think that the case in the absence of diversion message in INVITE.

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snom ONE (well in the standard configuration) keeps the from and to-headers the same. IMHO that is in accordance with the idea that the IETF had originally. In other words: From and To is what the user sees and they are not used for routing or billing purposes.


P-Asserted-Identity is used by the PBX to indicate who is paying for the call, and the user in there should be used for authentication purposes. But the PBX also supports ICID, which IMHO is extremly easy when it comes to billing (because the provider does not bill the local user on the system, but the trunk). Unfortunately, not too many people seem to use it.


In order to provide some evidence that this is not a spoofed call, the PBX uses the header "Related-Call-ID", which contains the Call-ID of the original call. The PBX is a B2BUA and changes the caller-ID for a redirected call. As a provider, if you provide service for both original and diverted call, you may check if the caller-ID has been spoofed or not (IMHO a great value to a client).

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Well, the PBX uses the header in certain cases e.g. when redirecting to an external mailbox. But not in the case of forking the call to the cell phone. IMHO it is debatable if this is a diversion, because the desktop phones might also be ringing at the same time. But I dont see how a Diversion header would hurt, so maybe we need to add this header as well.

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