You can use the Shortcuts app on iPhone to create a speed dial shortcut on the home screen.
To add speed dial on iPhone home screen, launch the Shortcuts app and follow the steps to select a contact and add it to the home screen for quick calling.
To add speed dial on iPhone lock screen, access the lock screen, tap 'Customize', select 'Shortcuts' in Widgets, drag the speed dial shortcut to the widgets section.
These steps provide a simple way to add speed dial functionality to both the iPhone's home and lock screens using the Shortcuts app.