[Asterisk-Users] transfer call after advise

[nik600]

i am developing a web application to manage callcenter, i will shortly
release it on sf.net

this is my problem:

i will grant to users the possibility to transfer calls to other users
using a web interface,

for example if SIP/200 is talking with SIP/400 who wants to transfer
the call to SIP/500 i use this commands with manager API:

Action: Redirect\r\n
Channel: SIP/200-sads\r\n
ExtraChannel: 500\r\n
Exten: 500\r\n
Context: from-internal\r\n
Priority: 1\r\n\r\n

this works fine (maybe the sintax now isn't correct... but it works),
but my problem is that the call is immediately transferred to 500.

I'd like if:

1 - 200 calls 400
2 - 400 want to transfer the call to 500
3 - 400 asks 500 if 500 wants to talk with 200

if 500 hangsup 200 still talk with 400
if 400 hangsup 200 talks now with 500

is it possible?
thanks nik