Hi Sian, If the phones been delivered it should have come with an activated SIM Card. Put the SIM Card in the phone and set it up and then look in settings under about phone and it will show your number there.
I don't know my account number either. And to get to the settings I need my phone number which I don't know. I did get an welcome email n it asked me my account number n my phone number which both I don't have.