How to configure Yealink T28 to support RFC 4235

[Flora_Yealink]

Hi ,
Yealink phone support RFC 4235 and for the question " when register, the phone won't send a SIP SUBSCRIBE " message , sorry , I have checked RFC but I didn't found it mentioned that the phone should send the subscribe message which include the event:dialog when the phone register,would you please share me the details ?Please let me know why you want the phone to send the subscribe mesage include " event: dialog" when register that I can check.
And Yealink phone do send the subscribe message include " event :dialog " when the account has configured BLF keys to monitor extension number, is it what you want ?

Thanks.