conference switch call

adou
Joined: Tue 23 of Jan, 2018

conference switch call

Posted:Tue 23 of Jan, 2018 (08:55 UTC)
dear all:
Existing A B C D
B and C is outging from PSTN
A and D Register with ippbx
A Place a call to the first party B.
A Press HOLD to place the first party B on HOLD.
A Place a second call C
Once the second party C is on the line, press CONF to start a three-way conference.

now,I want to implement a requirement like this:
To switch a conference call from the A to the D:
make B C D to start a three-way conference
What should I do about this?

Thanks & Regards