Accessing Speakerphone during a call

On an iPhone 13 How do I access the Speakerphone during a call when the white screen with keypad is showing, not the dark screen with the keypad and options showing? This happens when, for instance, I need to input numbers for whatever reason during a call. I select Keypad at the bottom, the screen then turns white and shows a keypad and a phone icon with some options across the bottom, but no option for Speakerphone. I need to get back to the dark screen with the Speakerphone option.