Skype for Business, Call controls do not appear in call window
If you are using a USB-tethered IP phone, call controls (such as transferring calls, muting your handset, etc) will be unavailable in the Skype for Business desktop call window. Please use the call controls on the IP phone's handset. If you would like call controls to be available on the Skype for Business desktop client, you will not be able to use the IP phone.
This page discusses reasons call controls may be unavailable in the Skype for Business client