Question about Sennheiser Pxc 550

Minimum volume with iPhone 6s and iOS 13.1. At work I like music very quiet and discreet in the background. If I connect the headphones via BT, there is something like a minimum volume level which, if you go below it, automatically switches to silent. It works better with a cable. Does anyone know a trick to get around this? Servus P from B.