[Asterisk-Users] Re: Grandstream transfer into outer space

[Stephen R. Besch]

Olle E. Johansson wrote:
> Going back to the subject, what does the grandstream really do, 
> SIP-wise, when you press
> the transfer button?
> 

Olle,

The following is an exact transcription of the description given in the 
BT101 manual for Blind Transfers:


4.3.7 Call Transfer The user can transfer an active call to a third 
phone by using the “Transfer” button. The sequence is like this: The 
user presses the “Transfer” button and if the other voice channel is 
available (i.e., there is no other active conversation besides the 
current one), he/she will hear a dial tone. He/She can then dial the 3rd 
phone and then hangs up his own phone. 2 kinds of blind call transfers 
are supported: using REFER and using BYE/Also. The SIP message flow 
based on SIP REFER method looks something like this:

Call Flow Diagram For Blind Call Transfer:

  From Transferee to Transferor

	 INVITE ->
	<-100/180/200
	 ACK ->
	<- RTP Media ->
	<- REFER
	 202 ->
	 NOTIFY ->
	<- 200
	<- BYE
	 200 ->

 From Transferee to Recipient

	INVITE ->
	<-  100/180/200
	 ACK ->
	<- RTP Media ->


The SIP message flow based on BYE/Also  method looks something like this:

 From Transferee to Transferor

  	INVITE ->
	<- 100/180/200
	ACK ->
	<- RTP Media ->
	<- REFER
	501 Not Implemented ->
	<- BYE with “Also:”
	200 ->

 From Transferee to Recipient
	INVITE ->
	<- 100/180/200
	ACK ->
	<- RTP Media ->

I have no idea if this is accurate, I just copied it and replaced the 
arrows indicating direction with "->" and "<-". You can download the 
manual itself from the GS web site.

Stephen R. Besch