Hello Nicole.Gray,
The issue here is the previous owner or his/her provider put an administrative password on the phone and unfortunately Cisco Support does not know those credentials to assist with the factory default of the unit.
The previous owner would have to provide that information for you to factory default it, or the previous owner's service provider that may have provisioned the SPA303 will have to supply the previous owner of those credentials so he/she could provide them to you.
I hope this help.
Regards,
Michael D.