[VIP-254NT] How to make 3-party conference call?

1. A4: A and B are in the process of conversation.
2. A want to invite C to join their conversation.
3. A press "Transfer" button to hold the conversation with B, and input the number of C (Follow by the "#" key).
4. C will ring up and pick up the handset to conversation with A.
5. A press "Conf" button and they will entry the 3-Way conference mode.