IMO, if a 12 year old child wants to talk to their father you have no right to prevent this, so if things happened the way you wrote, you did the right thing.
However, I do question why your son would be upset and angry when calling his dad. Of course, from your POV he certainly had every right to be so. But maybe things would have gone different if he had just asked his father to see him once in a while, rather than accuse him for not doing so? Of course, the interaction between a child and a parent is the parent's responsibility. Still, if a father doesn't do the right thing under all circumstances, maybe he would under better ones?
This is the part where I would suggest you question your role in this. I have been divorced (twice, actually) with children and know full well how hard it is to defend the very person who ruined your relationship (because its always the other one, isn't it?) in front of your children in order to not to make them feel bad about one of the two persons they look up to most. But it's still the right thing to do, and if you fail, your children will have to compensate for this.
(Note: I do not say you did, nor, if so, you are a bad person. But if you ask if there is something you could have done better, I would look at this.)
The way this turned out (and no matter whether you could have done better or not), it is now a pretty bad situation for your children: Their father refuses them. In the long run, this has the potential to seriously affect their psychology. If I was you, I would focus on these things to deal with the situation:
Do whatever you can to compensate for that situation. Talk to them about it. Do you live with someone else who can partially replace their father? If not, do you have a (male) friend who would like to spend time with your children, thereby doing some of the work their father ought to be doing? (Do they have a godfather?) If you think it is affecting them, consider a therapist. But make sure they don't feel like you expect them to break down under this. Children are very receptive that way.
Try to not to make your son feel upset about his father. I would not suggest that he'd apologize, but maybe if he'd try to talk to him again without anger (much later, of course), the two of them do have a chance to reconcile? That would be very good for your children.
Try to fix it. If your husband doesn't call back and tries to fix the damage he's done, maybe you can talk to him? And if he would not to talk to you, maybe there is some (common) friend he would listen to? Or do you think you could talk to your ex-mother in law? Would an uninvolved professional (external council) be an option? They are his children, after all, and I would wonder if he had no interest at all in their wellbeing – and this is threatened by his refuse to talk to them.
Hopefully, with time, this improves.