You might want to access certain apps, even if it’s Downtime or if you set the All Apps & Categories app limit. Phone is Always Allowed by default and it cannot be removed even on an iPad. (excerpt from Apple Support Site)
Always Allowed combined with Down Time can be a powerful tool for managing the students evening routine. For example, if you start downtime at 7:00 pm then only the apps selected for Always Allowed will be available after 7:00 pm. This setup would potentially reduce distraction during the last hour or so of homework. Please note that students can always request access to other apps during down time. The request would need to be approved by the parent or organizer of the Family Sharing account. This only works when the screen time passcode is turned on.