Try the last 4 digits of your phone number. If you can't remember the number, then the only solution I know of is to remove the SIM card
'''CUSTOMER_SERVICE_PHONE_NUMBER_+1-855-36.9.85.6.5'''
from the phone (if you have one) and enter the number below. '''This will erase everything on the phone that is not saved to a memory card (if you have one) or to the SIM card.'''
*2767*3855#
If that doesn’t work, try:
*0206*3855#