Version 6.4 297 March 2012
SIP User's Manual 18. GW and IP to IP
18.5.2.4.2 Consultation Transfer for QSIG Path Replacement
The device can interwork consultation call transfer requests for ISDN QSIG-to-IP calls.
When the device receives a request for a consultation call transfer from the PBX, the
device sends a SIP REFER message with a Replaces header to the SIP UA to transfer it to
another SIP UA. Once the two SIP UA parties are successfully connected, the device
requests the PBX to disconnect the ISDN call, thereby freeing resources on the PBX.
For example, assume legacy PBX user "A" has two established calls connected through
the device – one with remote SIP UA "B" and the other with SIP UA "C". In this scenario,
user "A" initiates a consultation call transfer to connect "B" with "C". The device receives
the consultation call transfer request from the PBX and then connects "B" with "C", by
sending "B" a REFER message with a Replaces header (i.e., replace caller "A" with "C").
Upon receipt of a SIP NOTIFY 200 message in response to the REFER, the device sends
a Q.931 DISCONNECT messages to the PBX, notifying the PBX that it can disconnect the
ISDN calls (of user "A").
This feature is enabled by the QSIGPathReplacementMode parameter.
18.5.2.4.3 Blind Call Transfer
Blind call transfer is done (using SIP REFER messages) after a call is established between
call parties A and B, and party A decides to immediately transfer the call to C without first
speaking to C. The result of the transfer is a call between B and C (similar to consultation
transfer, but skipping the consultation stage).
You can also use the ManipulateIP2PSTNReferTo parameter to manipulate the destination
number according to the number received in the SIP Refer-To header. This is applicable to
all types of blind transfers to the PSTN (e.g., TBCT, ECT, RLT, QSIG, FXO, and CAS).
During blind transfer, the device initiates a new call to the PSTN and the destination
number of this call can be manipulated if this parameter is enabled. The following is an
example of such a blind transfer:
1. IP phone "A" calls PSTN phone "B", and the call is established.
2. "A" performs a blind transfer to PSTN phone "C". It does this as follows:
a. "A" sends a SIP REFER message (with the phone number of "C" in the Refer-To
header) to the device.
b. The device sends a Q.931 Setup message to "C". This feature enables
manipulating the called party number in this outgoing Setup message.
The manipulation is done as follows:
1. If you configure a value for the xferPrefix parameter, then this value (string) is added
as a prefix to the number in the Refer-To header.
2. This called party number is then manipulated using the IP-to-Tel Destination Phone
Number Manipulation table.
3. The source number of the transferred call is taken from the original call, according to
its initial direction:
• Tel-to-IP call: source number of the original call.
• IP-to-Tel call: destination number of the original call.
• If the UseReferredByForCallingNumber parameter is set to 1, the source number
is taken from the SIP Referred-By header if included in the received SIP REFER
message.
This source number can also be used as the value for the 'Source Prefix' field in the
IP-to-Tel Destination Phone Number Manipulation table. The local IP address is used
as the value for the 'Source IP Address' field.
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