How to input from headphone mic, output to built-in speakers

My laptop (Kubuntu 18.04) has only a single sound card. I want to take input from the Mic of the headphone I connect but get the output to the built-in speaker of the laptop. I can toggle input (between internal and headphone mic) when headphone is connected, but if try to toggle output to speaker in the Pulseaudio Volume Control settings it shows "Unavailable" and if I switch to it, no sound comes out (i.e. the speaker works iff headphone is disconnected). I tried with Simultaneous Output but no luck.