How does the ZK-S9 Bluetooth headset work?

Hello,

although your request dates back 2 months, I am only responding to you now.
I received this same headset yesterday, and I managed to get it to work as follows:

First of all, charge the headset by connecting the mini-USB plug (it is charged when the red/blue light goes off)

To pair:
turn on BT on the phone, and start the "device search" mode
press and hold the central button for 5 seconds (you will hear a little sound in the headphones). Press again for 2 seconds (you will hear 1 note).
Normally, the phone will find the headset (it displays found device: ZK-S9)
Either it works, and you just need to confirm the connection on the phone and the 2 devices are paired; or you need to enter the code "0000" (4 zeros) on the phone and then "connect" to pair.

For the rest, the button referred to in the manual as "bluetooth key" is the central button; it is used to answer calls, hang up, turn off the headset,...