Only VoiceOver users have precisely this capability but all users can set audio to be directed automatically to the loudspeaker.
There is a switch which will ensure that all calls are hands-free with audio directed to the phone's loudspeaker rather than to the earpiece. Go to Settings / Accessibility / Touch / Call Audio Routing and select Speaker. If this switch is turned on and you want to take a call more privately using the earpiece, you will need to dismiss the keypad on your phone screen by tapping the Hide Keypad button and then tap the Audio button and finally select iPhone.
VoiceOver users have a more flexible choice. They can choose to have the audio directed to the earpiece when the phone is held to the ear or to the speaker when the phone is held away from the ear. You do normally need to begin a call with the phone held to your ear, though, and then you can immediately hold it away from your ear if you wish. The relevant switch is at Settings / Accessibility / VoiceOver / Audio / Auto-select Speaker in Call. This setting takes effect only when VoiceOver is turned on.
The Apple Accessibility team has submitted a feature request on my behalf to extend the fully automatic feature to non-VoiceOver users.