Resetting SIP-T42G

Purchased 3 phones from a re-seller. Unable to factory reset SIP-T42G phones. Even after holding "OK" button for 10 sec, the "admin" password does not work. Info for the phones is:

Phone 1 - Front Desk
IP - 10.1.13.18
MAC - 00:15:65:74:2a:4c
Firmware - 29.71.193.7

Phone 2 - Room 1
IP - 10.1.13.17
MAC - 00:15:65:74:30:52
Firmware - 29.71.193.7

Phone 3 - Room 2
IP - 10.1.13.19
MAC - 00:15:65:74:2a:24
Firmware - 29.71.193.7

Any advice on how to reset the phones? Thank you.