<human>: Can you give me the gist of the text in a nutshell?

Not sure if this is the right place but it involves my sisters relationship?

Known her my entire life (she's older so yeah) 

Ok. We recently got out of a family plan on our cell phones and haven't found a new provider so we're basically only able to make calls on the house phone right now. Earlier tonight, I picked up the phone to call a friend for a ride.

I didn't know my sister was on it and I would have started dialing had I of not heard her boyfriend screaming at her. He was saying things like, "If you don't talk to me, I'll snap your neck," or "Don't think I won't beat the shit out of you again," etc etc etc. 

I know I should have hung up. Something. But I didn't. I listened for about 10 minutes before I couldn't take it anymore and slammed the phone down. She heard this and about five minutes later was in my room yelling at me for spying on her and that I can't tell our parents.

My question is, what do I do?! Do I go to our parents? She's an adult now, what can they do? I'm scared her boyfriend who she's only been with for about 4 months is beating her, or will beat her, and it terrifies me and makes me angry all at the same time.

When I told her I didn't like it, and asked if I could help, she told me to shut up and that it wasn't a big deal, but I'm really worried.
<bot>: Listened in on a convo my sister was having with her bf. He threatened to kill her, beat her.