[asterisk-users] SIP REFER ans Dialplan

[kalle at esprITschmiede.de]

Hello!
I have a question to the asterisk dial plan.

I have the following situation:

1. There is a Sip-Call from Bob at my_Asterisk to Alice at WHATEVER
(Call-ID:10 at WHATEVER)

2. There is a Sip-Call from Carol at my_Asterisk.com to Ralph at my_Asterisk but
this goes through the Dialplan. So that there are actually two calls
Carol->Asterisk (Call-ID:20 at my_Asterisk) and Asterisk->Ralph
(Call-ID:21 at my_Asterisk)

Now Carol at my_Asterisk sends an REFER to the Asterisk with the objective that
the call Carol replaces Bob in the first call (Call-ID:10 at WHATEVER). She
does so by adding a Refer-To Header to her REFER Message:
Refer-To: <sip:Alice at my_Asterisk.com?Replaces=10%40WHATEVER%.....>

Bob, Carol and Ralph are regstered Users in my_Asterisk.

Now my question:

What might I add to the extensions.conf or sip.conf that Asterisk processes
this REFER?

Thank you very much, 

Kalle