This a Phase 3, Open-label, Randomised, Active-controlled, Parallel Group, Multicentre Study to Investigate the Safety and Efficacy of PA21 (Velphoro®) and Calcium Acetate (Phoslyra®) in Paediatric and Adolescent CKD Patients with Hyperphosphataemia. The aim of this Phase 3 clinical study is to demonstrate similar efficacy of PA21 (Velphoro) in paediatric and adolescent patients with CKD, and to provide safety and dosing information for this patient population. The Phoslyra (comparator) group provides information for a descriptive comparison of PA21 against a commonly used calcium-based phosphate binder (calcium acetate).