The physiological change occurs in a cardiac muscle cell when there is plateau phase of action potential is:

The transmembrane action potential of single cardiac muscle cells is characterized by, Rapid depolarization (phase 0) Initial rapid repolarization (phase 1) Plateau (phase 2) Slow repolarization process (phase 3) that allows return to the resting membrane potential (phase 4). The initial depolarization is due to Na+ influx through rapidly opening Na+ channels (the Na+ current, INa). The inactivation of Na+ channels contributes to the rapid repolarization phase. Ca2+ influx through more slowly opening Ca2+ channels (the Ca2+ current, ICa) produces the plateau phase, and repolarization is due to net K+ efflux through multiple types of K+ channels. Ref: (2012). Chapter 29. Origin of the Heabeat & the Electrical Activity of the Hea.In Barrett K.E., Boitano S, Barman S.M., Brooks H.L. (Eds), Ganong's Review of Medical Physiology, 24e.