Yes one of them at least will call out during the day and or night.
Some say it's just there way of checking out who is around. Though Like you've said they can and will do it in there sleep.
Even asleep they don't miss a sound and sometimes even the quiet may make them do it.
Maybe they are dreaming, hard to say, thing is I never hear anyone bark back to them.
It's always a lone voice, most of them will do at one time or another, and some just don't even wake to do it.
When I'm handy I just say back to them "yes I'm hear, now go to sleep."