Good advice from Cog and AF, as we'd expect :) I'm a bit scared of being a bull in a china shop here, so I do hope others will correct my mistakes.
Because it's so very important to listen, and to believe your caller, there can be a strong temptation to tell her what she's feeling. "That must have made you feel ..." is subjective, though - it's really about you - and can stymie someone who's conditioned to agree. In the same vein, "How does that make you feel?" is a demand. When you look at threads on here, experienced posters never do this. They ask more about the events - Where are you now, does he often lose his temper, what happened next, where is he now, are the children safe, etc. (Not all in a row like that!!) Sympathy comments tend to be limited to Oh dear, poor you, ouch, or somesuch.
Downtrodden women blame themselves: It's my fault, I'm useless, I'm as bad as him, it's six of one and half a dozen of the other, kind of thing. I shouldn't have set him off. Your organisation will have guidelines on this but, again, avoid acting like you know better. "You're probably not useless" and "It might not have been your fault" could be safe responses.
Don't be afraid of silence! I once spent an hour, tongue-tied, on the phone to a Samaritan who just said evenly, every few minutes, that he was still listening and I could talk when I was ready. Towards the end of the hour he told me I could ring back and ask for him, if I wanted. I did - twice in a row, and I was a lot less silent for the next 2 hours 
What he did, very well, was provide a safe space that was purely for me. When I'm trying to do this for someone else now, even face to face, I always think of it as 'keeping the line open for them' as he did for me.
On behalf of too many women in painful circumstances - thank you for doing this.