Welcome to the community!
Thanks for raising this to us. We do not have information on how to setup Call Diversion on Yealink Phone Handset, we'd like to confirm if you're with TPG Bizphone service, please send us your account details (Username/Customer ID together with the address on file). We'll check this for further investigation.
In case you need a reference:
How do I divert all three lines of my Yealink phone to a mobile? I have managed to divert one phone line but cannot work out how to pick up the other phone lines to divert them.