Sennheiser ACCENTUM Over-Ear Wireless Headphones (Black) - Manual (Page 14)

Using the headphones
14 | ACCENTUM
Connecting the headphones to a
Bluetooth device
To be able to use the Bluetooth connection, you must register both
devices (headphones and e.g. smartphone) once. This process is
called pairing.
If operation differs from the steps mentioned, also refer to the
instruction manual of the Bluetooth device you are using.
If you switch on the headphones for the first time (the pairing
list is empty), they automatically switch to pairing mode.