Hi!
In order to use the Bluetooth function, you need to first pair your device to the headphones; the flashing white light indicates that the headphones are ready and waiting to pair with a Bluetooth-enabled device, like your phone or laptop. [https://www.jbl.com/on/demandware.static/-/Sites-masterCatalog_Harman/default/dwfa63e1bb/pdfs/E40BT_QSG_EN.pdf|This is where you should find instructions] on how to set up your headphones.