The SP series can be utilised as a hands-free device with certain deskphone models from Cisco and Snom. (Sennheiser only guarantees full functionality with Cisco 8961, 9951 and 9971.)
In order for this to work correctly, the correct mode must be selected on the SP.
By holding down the 'end call' and the 'mute' buttons the central buttons will begin to flash, showing first which mode the SP is in, and then changing mode.
Flashing red indicates that the SP is in PC and softphone mode.
Flashing blue indicates that the SP is in Cisco (/Snom) mode.
By repeating the process the SP can be toggled between these two modes.
It is important to note that should the SP be in Cisco mode, it will not support call control when connected to Skype for Business or to HeadSetup.