Hi Krishna,
When the call comes in only the accept button will be active and the rest of the buttons will be inactive or will not function. Even if you see them active but if you click on any of the icon other than accept and reject call button during the call comes in , system gives the message "you must be in active call to use ................".
If you still want to configure the button functionalities based on the state of the call (like during the busy or during the idle) you can check the settings in the transaction CRMC_IC_TLBPROF and go to the telephony layout buttons and you can control the button functionalities by activating or deactivating "loggedoff" , "idle" and "busy" check boxes.
Regards,
Kumar