It might not be immediately obvious, but on WhatsApp at least, you can hold your phone to your ear and the voice clip will play as if you're on a phone call. I am making an assumption that Signal would be the same.
I always thought this was an iPhone feature, not strictly WhatsApp. Back when I used WhatsApp I tried this with my Android phone and couldn't get it to work. Definitely possible that I overlooked something crucial though.
