BLF State Indicator "off" and Asterisk 16

[terraformer]

Device: T48S
Firmware Version 66.85.0.5
Hardware Version 65.0.0.2.0.0.16

Hello,
I'm using some BLF keys and, in general, they do work in combination with Asterisk 16. Changing the DEVICE_STATE to NOT_INUSE | INUSE | BUSY | RINGING | RINGINUSE | ONHOLD (see Asterisk wiki) does indicate the expected state, cf. Yealink T4 Series IP Phones User Guide V85.1, p.60. DEVICE_STATE UNKNOWN does not affect the BLF which seems reasonable.
However, DEVICE_STATE INVALID | UNAVAILABLE changes the BLF to solid green (i.e. "idle line") which I consider incorrect.
So, how can I change BLF to "Off" state, like shown in the User Guide on p. 60? Or is it a bug?
In "Yealink SIP-T2 Series T4 Series ... IP Phones Administrator Guide V85.1" on p. 436 is shown a SIP NOTIFY message. Any chance someone can tell me the content that changes the BLF to "off"?

Thanks & best!

[terraformer]

Hi, what is the purpose of your post? It does neither answer my questions nor is it related to the content. Looks like you are just advertising your website.