Top-bar main menu is a big screen as bottom-bar main menu is an small phone screens. Here is an example of how to build a smartphone interface with multiple interfaces. I just added two interfaces the first screen has a welcoming message. The second on a keyboard to dial.

The buttons controls the screens through the ScreenManager and the current property. All we have to do is assign a a name to the screens and change `current` property to the desire screen when the `on_press` event is triggered, like this:

    on_press: _screen_manager.current = 'screen1'

_screen_manager is just a kivy id so we can reference it (the ScreenManager)inside the Kivy Language. So, here is the code.

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.floatlayout import FloatLayout

Builder.load_string("""
<Phone>:
    AnchorLayout:
        anchor_x: 'center'
        anchor_y: 'top'
        ScreenManager:
            size_hint: 1, .9
            id: _screen_manager
            Screen:
                name: 'screen1'
                Label: 
                    markup: True
                    text: '[size=24]Welcome to [color=dd88ff]THE APP[/color][/size]'
            Screen:
                name: 'screen2'
                GridLayout: 
                    cols: 3
                    padding: 50
                    Button:
                        text: "1"
                    Button:
                        text: "2"
                    Button:
                        text: "3"
                    Button:
                        text: "4"
                    Button:
                        text: "5"
                    Button:
                        text: "6"
                    Button:
                        text: "7"
                    Button:
                        text: "8"
                    Button:
                        text: "9"
                    Button:
                        text: "*"
                    Button:
                        text: "0"
                    Button:
                        text: "#"
    AnchorLayout:
        anchor_x: 'center'
        anchor_y: 'bottom'
        BoxLayout:
            orientation: 'horizontal'
            size_hint: 1, .1
            Button:
                text: 'Go to Screen 1'
                on_press: _screen_manager.current = 'screen1'
            Button:
                text: 'Go to Screen 2'
                on_press: _screen_manager.current = 'screen2'""")

class Phone(FloatLayout):
    pass

class TestApp(App):
    def build(self):
        return Phone()

if __name__ == '__main__':
    TestApp().run()

Hope somebody found it useful.