Transferring a call with a Yealink phone is easy and can be done in 2 ways.
The first way is that you answer the call and then start transferring the call to a colleague.
You first want to speak to your colleague and pass on who is calling ("I have Kees from the company Shop on the line").
The following steps should be taken:
Answer the call
Press "transfer." On the newer models this is a button under the screen of your phone
You can now start a new call to your colleague. Enter the number of your colleague, for example extension 303. Press "send" (or "dial").
Your colleague answers your call
You tell him who is on the line
Press "transfer".
The call is now transferred to your colleague and you can hang up.
A faster way is to transfer the call "cold" (or blind).
You then don't tell your colleague who is calling, but transfer the call and so you don't speak to your colleague first.
Answer the call
Press "transfer".
Dial your colleague's number, for example 303. Press "transfer".
Done !