When in charge of the re-design of the VoIP call screen of a mobile application I started by mapping the whole VoIP call scenarios, from launching the call to ending it, going through all the possible use cases it implies: having sufficient credit, establishing network connection, calling an unsupported number, redirecting to in-app calls, receiving no recipient answer and so on.

Once the map was clear, it helped understanding which information was useful in each moment of the process and therefore organise its hierarchy. The proposed call screen reflected these results in order to display only the elements needed in each state.