It is possible to control calls using R/ Flash button on W02FXS 2018, W04FXS 2020 and W24FXS 2015-2017.
In case of the second incoming call during an active call:
In case you have one active call and one call on hold:
In case you have one active call:
generate a new call
Note: this step can be skipped in case of a second incoming call (this case is already described in the documentation).