How do I completely reset this modem back to factory defaults?

There is no command that will totally reset the modem back to it's factory default settings. The AT&F command will default many of the parameters, but not all.

Restore Factory Settings&F

Description: This command is used to restore the factory settings from EEPROM. It only restores the parameters that can be found in the Parameters Storage table with column AT&F checked. Those parameters are restored in RAM and in E2P, overwriting the profile for AT&W.

Note: This table can be found in Appendix A in the manual.

Command syntax: AT&F<cr>