Do I phone the police and tell them my dad is beating my mum?

Sometimes my dad drags my mum upstairs and locks the bedroom door. I sneak up and listen outside. I hear him saying nasty things to her like "you've been a dirty little tramp and need to be punished". But instead of my mum fighting back she says something like "I've been a very girl and deserve a punishment I won't forget soon". They also say other things but they're too obscene to say on here. Then I hear banging coming from inside where my dad is hitting her, but when they come out my mum is smiling. I can't quite understand it. Shall I call the cops on him next time he does this to her?