Great advice here and I don't know if the following will help at all but - one thing you can do is look to the future and KNOW that you will be able to sit your boys down, look them in the eye and justify each and every one of your actions during this difficult time. And they will be able to see and understand exactly why and how you put them first, prioritising their relationship with their dad, upholding the contact even when they (at 7 and 3) didn't want it. When they're grown up, they'll understand and admire you for your approach. And they'll know that you always, always put them first. As young adults, the security and confidence that that knowledge will give them is more precious than rubies.
It doesn't sound as if the same will be true for your ex, unfortunately. Sadly, when it comes to situations like this, nasty abusive bullies do get their comeuppance... it sounds as if unless he changes his attitude to YOU quite radically in the next couple of years, your boys are going to vote with their feet as soon as they're old enough. And it WON'T be your fault. Your support now, although it must be tearing you apart, will at least give you the peace of mind to know that if they made the decision to not see their dad in the future, you can put your hand on your heart and know that you had nothing to do with it. Similarly, if their waste of skin of a father can muster up enough humanity to provide them with at least a shadow of what they'd want from a father, enough to keep them wanting him in their lives, you can take credit from that result too.
Of course all of the above only follows as long as the situation doesn't tip over into worry. As others have said, please keep your diary, document every abusive action on his part, especially those in front of the boys, and report report report. Keep an eye on the boys and be ready to take it back to court if necessary.
Remember that with every abusive move (such as not letting your child speak to you) your twat of an ex is only actually harming HIMSELF. Showing his two sons what a horrible bullying monster he is, and making them ever more likely, with every snarl and push, to ensure that as soon as they're old enough they'll not want to see him. More fool him!
Meanwhile, you have succeeded in moving on to an everyday life where his abuse and bullying is a distand memory, contact notwithstanding. Where the boys have a happy home life protected from this bully. They'll grow up better and happier than they would have been, because you have provided them with a safe loving hom away from him.
Sending you all strength.