1st. Put the existing call on Hold. Then open another line by clicking the Dial Pad.
A Dial Pad will appear.
2nd. Dial the phone number and then hit the "Call" button. If you have more than one SIP accounts, you will see another dialog box for you select which SIP account (phone queue) you will be dialing from.
3rd. Once connected. Do the "Warm" transfer introduction by introducing yourself, giving the caller's details and reason for transferring and when the other party agreed to take the call, go back to the 1st window, click the Transfer button, on the drop-down select the number that you are going to transfer the call to.