Contact us immediately. You'll find our contact details by clicking the contact us link on this page. Have your IMEI and SIM numbers available when you call. These are included in the information provided with your handset. We will then permanently bar your phone so that it is no longer possible to use it to make any calls.
If your phone's been lost or stolen it's really important that you get in touch as soon as you can because you will have to pay for any calls made from your phone, up to the moment you report it missing.