By Default the Yealink Phone does a blind transfer and is not optionable for Consultive.
To do a consultive transfer, you must do the following using the Handset (not Speaker Phone)
1) You have a call you want to transfer, put that caller on Hold (not Park) using the phyiscal HOLD button.
2) Choose a second line and either dial your number or use the speed dial
3) When the person answers, ask them if they will take the call
4) If they do, press the TRANSFER button and you are done.
5) If not, just hangup on the person you called and go back to the line on hold.