If you go to Options #2, then Advanced Options, under COMs there's an option named 'Auto Clearance Coms'. If you hover your mouse over the question mark (help) icon you should see the following text, which, I think answers your query
With this option selected, after you've made initial contact with ATC to request clearance, all other radio calls will be performed automatically by your CP, provided of course you have your CP mode set to 2 or 3. With this option OFF you will have to manually press Hotkey #0 to request either 'Push & Start', 'Start' or 'Taxi clearance' when instructed to do so. Some users prefer to have it this way so they can decide when they're ready to continue to the next stage.