Bioactive phosphate-based glasses (PBGs) have several possible biomedical applications because of the chemical reactions they undergo with their surroundings when implanted into the body. The dissolution rate of PBGs in physiological conditions is a crucial parameter for these applications, to ensure, e.g., delivery of drugs or nutrients to the body at the correct rate. While it has been well-known that increasing the CaO content of these glasses at the expense of Na2O slows the dissolution rate, this paper provides an atomistic explanation of this for the first time. In this work, molecular dynamics simulations of five ternary P2O5-CaO-Na2O glasses reveal the structural properties at the atomic level that enhance the durability of PBGs as more Ca is added: (i) Ca binds together more fragments of the phosphate glass network than Na, (ii) Ca binds together more PO4 tetrahedra than Na, and (iii) Ca has a lower concentration of intratetrahedral phosphate bonding than Na. This behavior is rooted in the calcium ion's higher charge and field strength. These results open the path to precise control and optimization of the PBG dissolution rate for specific biomedical applications.