Child Maintenance Payments

Hi, my ex partner who isn’t due to make his first payment until next month has already recorded 3 change of circs by falsely claiming he has our daughter when he doesn’t in an attempt to reduce his payment. How does this work? Will CMS just take his word for it? I’ve tried calling them but I’m being told to use my online account which tells me nothing! All I can see is what the payment amount is, not when it’s due, how they got to that amount, nothing about the changes he’s reporting and I can’t view any of the messages I’ve sent to CMS asking for an update.
Any advice would be greatly appreciated. Thank you x