YES! If Fialer is not made your phone's default phone dialer, the dialer codes will not work.
The first time you open Fialer after downloading, you will see a popup that asks if you want to set Fialer as your default phone app. Choose Fialer and "Set as default." If you did not do this, search for "Default phone app" in your Android settings and choose Fialer. But don't worry, even though Fialer will be your phone's official dialer, we have added some special magic to Fialer that will allow you to continue to use your existing dialer just like you always have!