I guess it's not rising to any 'bait' and not showing emotion. Not letting him get a rise out of you or see that the things he says bother you. Unfortunately it involves biting your tongue and not letting him 'have it'. And that's really hard.
Remember that nothing he says or does is worth your time. If he says or wants XX and it doesn't work for you so you say no and he says 'why can't you be nice' or 'don't be nasty' simply do not respond. And don't be tempted to fill an awkward silence:
Him: I want to talk to DC
You:They're settled in bed now and I don't want them riled up. You'll need to call earlier tomorrow. (said calmly but assertively)
Him: I said I want to talk to DC, put them on the phone. Why are you always so nasty?
You: You'll need to call earlier tomorrow
Him: God you're a bitch
You:
Him: Are you there?
You: Yes. But I think it better that we terminate this conversation. Good bye.
Then block his number for the rest of the night.
With you living in another country, how much interaction do you have to have with him? And about what? If it's over money and you aren't able to pursue it legally, then you may as well save your breath. If it's about visits, if there are orders then you must abide by them. If not, you can do what you want (and is best for DCs).
The thing you need to remember is that nothing you say is going to affect him in a way that will make him change. So why bother? Why put yourself through the stress and aggravation? And remember also, that you do not have to speak to him for anything other than the children. When he veers off into attacks, hang up. Keep hanging up until he learns that you will not listen to verbal abuse. But, again, for this to be effective, you can't indulge in verbal abuse either.