Make sure your iPlum account has the phone tree setup on main line. After you receive a call on any extension, you can transfer that call to another iPlum number or external number by pressing *7 (star 7) and entering the 10 digit phone number followed by #. Then after the call is connected, press:
0 to conference.
1 to exit yourself.
2 to end call transfer.
A calls B. B wants to transfer call to C. (C - Phone Number: 202-123-1234).
After A gets connected to B, B will enter *7 in their dialpad followed by 2021231234#.
A will hear Music-on-Hold.
B can talk to C for one-one conversation, then press 0 to conference C into the call with A.
Or press 1 to take itself (B) out of the bridge and let A and C have the conversation.
Or press 2 to end the call with C (keep conversation between A and B).