When my daughter's phone, a Rumor 2 purchased through Sprint, started rebooting randomly and not always recharging we took it to the Sprint store. A battery swap and call to technical support later and we were issued a warranty replacement through the mail. Before sending her defective phone back I wanted to make certain all contacts and other data were removed. I knew the factory reset option was in the security section. A "lock code" was requested to access security, but neither my daughter, a Sprint customer service rep nor I knew what it was. A couple of days of searching I found it. I pass the info along here in order to make this simple task easier for others.
The path to reset:
Menu > Settings > More > Security > Enter Lock Code > Delete/Reset > Reset Phone
What's the "Lock Code"? It's the last four of the number associated with the phone. In other words, if the number assigned to the phone is 212-555-0020 then the "Lock Code" is 0020.
Got it? Simple, right?