M3902 has the conference or transfer mapped to button 4, a fixed key reserved for either conference or Transfer. It is not usually placed on any other button. Only, the three soft keys are supposed to be configured as programmable buttons. Note that for the 3902, the soft keys are not context sensitive.
M3901 allows the button to be configured as button 1, 2, 3, 4, or 5. It is accessed by pressing the feature key and then 1, …, or 5 as per the configuration.
The remaining stations allow the user to configure the conference service on any programmable button.