Deliberate mistake: Geologically, the type of volcano in the movie shouldn't erupt so much low-viscous magma; but due to make the volcano exciting to the audience, the movie makers probably deliberately ignored this mistake.
Deliberate mistake: In the shot right after Harry successfully maneuvers out of the river, the truck he's driving should still have water pouring out of it. When he was initially pulling out, the water in the truck was still up to the bottom of the steering wheel, and we can see it draining out, but it would take much longer than a second or two for the water to fully drain out- the truck's completely dry and not leaking at all. (01:03:40)
Answer: He's young and he and his sister are alone. He's worried about his Grandma and she's not answering the phone. He's saying hello because he desperately wants her to answer so he knows she's OK. He feels helpless just letting the phone ring.
The_Iceman