[Users] Accounting Information and Call Transfers

[Günther Starnberger]

Hello,

I'm currently working on a VOIP network where the phones register to an
OpenSER server - we also have a PSTN Gateway (Asterisk).

To be able to account the PSTN calls of a user we insert a custom header
"X-Account" which stores information about the user account. We can't
just use the From-Header for this as we allow users to e.g. create
unconditional forwards which forward the call to a defined SIP uri when
the user isn't available. If somebody creates a forward to the PSTN it
is still his account which should be accounted and which is therefor put
in the X-Account header.

This part works fine, but I'm still not sure how to handle call
transfers - e.g. when a user wants to transfer his conversational
partner to the PSTN. An unattended transfer in SIP is implemented with a
REFER request which causes the remote phone to send an INVITE to the SIP
uri in question[1]. I'm not sure how to account these transfers. On a
traditional PSTN network the person who caused the transfer would be the
one who is paying for the call. But I see no solution how to implement
this behaviour in SIP (because the old call isn't transfered, but the
conversational partner starts a new call with the INVITE request).

Is it somehow possible to detect if a call was caused by a transfer so
that I can insert the right X-Account header? Do I need a B2BUA like
Asterisk to implement the traditional behaviour? How do other VOIP
providers solve this problem (IMO call transfers shouldn't be such an
uncommon feature).

tia for all suggestions on how to solve this problem
/gst

[1]
http://www.ietf.org/internet-drafts/draft-ietf-sipping-service-examples-08.txt

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