You'll have to forgive at least a little bit of his confusion; he doesn't tend to receive calls long after he's gone off to sleep.

Izuku wipes the drool from his lips and stares, blankly, at his phone...Before he realizes who's calling, and after squinting his eyes for a few seconds, his braincells slowly connect. Dumbfounded, a little flustered and perhaps also panicking, he picks up the call with a blush on his face, though still unluckily sounding slightly sleepy and groggy despite his attempts. “Hey...? What are you doing up so late?”