That's possible now and you don't even need a shortcut. Just say "Hey Siri, call Mom on speakerphone". Has been around since 2018. start-a-speakerphone-call-with-siri-iphoneExactly that. Shortcut only allow you to call someone but not put the audio through phone speakers which for some contact - like calling my mother - I’d have to select speakers every time which is tiresome.