I don't have children, so I'm probably not the best person to help, but to me what she's doing seems very unreasonable.
Please someone jump in if this is way off the mark...my immediate thoughts would be to state that the way things are at the moment seems to be causing your XW some distress, and explain what you said, about it being hard to get the kids to sit down at the phone at exactly the right time.
Could you propose a 10 minute/15 minute/20 minute (Whichever you prefer) time slot during which you'll call back?
