One of your first orders of business upon activating a new wireless handset is learning your new phone number. If you live in the UK and use O2 as your wireless provider, there are several options at your disposal to help you find your phone number--even if you have no calling credit in your O2 account.
Call O2 customer service and ask the representative to tell you your number. Even if you don't have mobile credit, you can still make calls to official O2 numbers. Dial 4445 from your phone to be connected to customer service.
Sign in to your O2 account online to access your phone number online. Click the first link in the Resources section to access the login page, for which you will need the user name and password you established when you set up your O2 account.
Consult any old O2 bills or documents you've received in the mail to find your mobile number. The specific location of the number will vary, depending on the document in question.
Ask any friends whom you called when the phone did have credit to peruse their phone books and/or "Received Calls" list to locate your number for you. If you called anyone with a cell phone or "Caller ID" service on a landline, they may have your number saved.