The function of Phospholamban is
Phospholamban inhibits calcium pump on sarcoplasmic reticulum, thereby preventing uptake of Ca++ from the sarcoplasm. 
When it is phosphorylated, the Ca++ channels open up and uptake of Ca++ is increased into the sarcoplasmic reticulum.
Regulates Na+ K+ pump
Transports calcium out of the mitrochondria
Binds actin with myosin
Inhibits Ca++ pump on sarcoplasmic reticulum