I found a glow-in-the-dark tail cap button for my BLF-A6! : r/flashlight
Amazon.com: TIE 36 BLF Exec 15618 12 Button Electronic Telephone with Display : Office Products
Small Business VoIP: Enable Asterisk BLF (Busy Lamp Field) in Yealink and Grandstream IP Phones « Asterisk How To « Small Business Asterisk PBX | Amazon AWS EC2
BLF (Single Button) Transfers on a Yealink T54W Desk Phone - YouTube