Well if they don't call you back I would try leaving a detailed message that ends with "If I don't hear back from you by COB Friday this week, I will take that to mean the company is allowing me to retain all funds that the company inadvertently contributed to my plan." Or something along those lines that is in no way legally binding, but might motivate the HR person to get back with you.
Alternatively and my preferred method, if you still have contacts at the company get the personal work email address of the HR person in question and email them instead of calling.