You absolutely have to relay to them, any and all instances of abuses towards the DC. This is your only hope of protecting them.
I would draw yourself up a list of the worst incidents, so you can relay what happened and how the DC affected.
It is futile to express your anger to the person listening/recording the call, as they need to hear very specific details.
Do not judge yourself on his behaviour as being you slagging him off....its either true or it isn't, its not 'slagging off' its evets as they happened...keep it factual. X did y to z, caused A & B in DC.
If DC are now bed-wetting,sleep disrupted, suffering nightmares, becoming increasingly anxious, struggling at school, become withdrawn, hurting themselves, wetting themselves, since 'when' then make sure you say so. This is about protecting your dc, not an opportunity to slag off the X.