Reproducible thermal phenomena for calibrating thermometers include the boiling and melting points of water, which serve as fixed points in the Celsius temperature scale. However, these points are pressure-sensitive, making them less reliable for calibration across different locations without specifying pressure conditions. The discussion raises concerns about the precision of using water's phase changes for calibration due to this variability. Additionally, the concept of phase changes that are not pressure-sensitive is mentioned, though specifics are not clearly defined. Overall, the calibration process requires careful consideration of pressure effects on thermal phenomena.