The local structures of a series of amorphous calcium phosphate (ACP) phases with increasing carbonate contents (2–14 wt %) were studied by multinuclear ¹H, ¹³C, ²³Na, and ³¹P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) experiments together with infrared (IR) spectroscopy. A model for carbonate incorporation into ACP is proposed, where carbonates enter as CO₃^2– anions, whose equal ¹³C chemical shifts (δ_C = 168.6 ppm) imply identical local CO₃^2– environments in the ACP structure, irrespective of its carbonate content. The bicarbonate contents were negligible, except in the CO₃^2–-richest ACP sample, where HCO₃^– ions accounted for 4.3% of all carbonate species. The HCO₃^– anions in ACP are characterized by ¹³C and ¹H chemical shifts δ_C = 162 ppm and δ_H = 14 ppm, respectively, as deduced from ¹³C{¹H} heteronuclear correlation (HETCOR) two-dimensional (2D) NMR experiments. Regardless of the precise carbonate content, the ACP samples contained very similar amounts of water (≈15 wt %)—most of which is structure-bound (≈70%) and the remaining physisorbed—along with acidic protons of HPO₄^2– anions, which typically accounted for ≈20% of the phosphate speciation. The local proton and phosphate environments were probed further by heteronuclear ¹H/³¹P 2D NMR experiments. We also extracted the ²³Na NMR parameters of the Na⁺ sites present in minute amounts (0.1–1.1 wt %) in the ACP specimens, which along with their ¹³C/³¹P/¹H NMR counterparts of the CO₃^2–, HCO₃^–, PO₄^3–, and HPO₄^2– moieties are discussed and contrasted with previous reports on Na/carbonate-bearing Ca phosphate phases, such as synthetic and biogenic hydroxy-carbonate apatite. The spatial distribution of the carbonate species was determined from advanced homonuclear ¹³C and ³¹P double-quantum together with heteronuclear ¹³C{³¹P} MAS NMR experimentation, where each technique provided independent and consistent evidence for randomly distributed CO₃^2– moieties.