This knowledge base article shows you how to transfer a call on the Yealink Handhelds.

When you are on the phone and you need to transfer to another extension within the building:

Push the Options soft button while on the call.

Click on Transfer.

Scroll to the handheld you want to send the call to.  Your caller will hear music while they are waiting.  The person you transfer to answers the call, and you can talk to them, then click on the transfer soft button to complete the transfer.

If the extension you want to transfer the call to is not listed in the listing, dial that person's extension number on your phone and press transfer.