You might start documenting all of this, especially when he DOES talk to them, in case his weird behavior extends to taking legal action because “you’re not making them talk to him.”
Is there any chance his new wife has some weird idea about wanting more custody? The timing is just so strange.
But heck, yes, let it go to voicemail. Then treat him the way you would one of your kids having a supermarket meltdown...firm and calm.
Sorry you’re dealing with this!