At which two temperature points does phase change occur?

Such is easily seen on the 'Heating Curve for Water' (see attached). Note the regions that have zero slope. Trace back to y-axis of graph. These points are the phase transition temperatures for freezing (Iiquid => solid)/melting of ice (solid => liquid) and boiling of water (liquid => gas) / condensation (gas => liquid).

Note also, during phase change (heating or cooling), two phases will always be present (solid & liquid) or (liquid & gas) and experience 0 temperature change during the transitions (zero slope regions). The regions having slope values are heating or cooling of pure single phase form of the substance and do not represent phase change in those regions.