I'm sure that dreaming requires sentience, in which case the answer would simply be "no".
Your brain is doing a lot of processing when you are sleeping. When a phone "sleeps" it means that it shuts down processes beyond those needed to track upcoming alarms and monitor connections for incoming signals, reduces clock speeds and turns off parts of the logic. So rather than doing something different when it "sleeps", a phone is simply doing less than when "awake". We use those anthropomorphic terms to describe when the phone is more or less active, but they are nothing more than very imprecise metaphors.