Is there any way you can screen your calls at work? Let them go directly to voicemail and immediately return any work-related calls? This way you would have the harassment documented.
I think you would have good grounds for filing a harassment complaint, which includes repeated communications at extremely inconvenient hours or with no legitimate purpose and with the purpose to annoy or harass. No reasonable person would expect another to leave work to drop off children in the middle of the day. It's clear enough why he's doing it. If he's convicted, the court could order no contact as a condition of probation.
I know it's exhausting and demoralizing. Maybe you could contact an advocate for moral support during this time.
Hugs,