What is the reaction mechanism for the hydrolysis of Amifostine?

I have to do research on a drug called Amifostine: 2-(3-aminopropylamino)ethylsulfanyl phosphonic acid.

I'm supposed to say how the organic reaction works inside the body. I understand that it becomes hydrolysed by an enzyme become the active form; 2-((aminopropyl)amino)ethanethiol.

I don't get what happens to the phosphonic acid functional group. Does the water break apart the S-P bond because it is a weak bond? And what does the acid group become? $\ce{H3PO4}$?