Sennheiser PXC 550-II Instruction Manual page 14

Using the headphones
14 | PXC 550-II
If you pair the headphones with the ninth Bluetooth device, the saved
connection profile of the least used Bluetooth device will be over-
written. If you want to re-establish a connection with the overwritten
Bluetooth device, you have to pair the headphones again.
Paired and connected Bluetooth device
Paired Bluetooth device (currently not connected)