When my husband's ex did this, he called every day and left a message on her voicemail. After about a week of this he then left a message telling her he was concerned since he hadn't heard from them and if he didn't hear from her within 48 hours, he was going to file a missing person's report on her and their daughter. She called back the next day.
I believe you said your ex's phone has been shut off, perhaps you could file a missing person's report on your children. If it's been long enough. Some might say that's a bit drastic... but you have to think of your children's well being. Good luck.
