If you need to report your mobile stolen the police will want your IMEI number for your phone. This can be found inside the phone usually behind the battery. You can also obtain it by dialling *#06# that is star,hash,zero,six,hash. If you still can't obtain it here is a list of contact numbers for phone companies
Operator Contract Pre-pay
Orange 07973 100150 07973 100450
Vodafone 07836 191191 08700 776655
O2 UK 08705 214000 08705 678678
T Mobile 08454 125000 08454 125000
Virgin Mobile 08456 000789 08456 000789
3 Mobile 08707 330333 08707 330333
Onetel 08009 571100 08009 571100
Fresh Network 08701 115000 08701 115000
Tesco Mobile 08709 004455 08709 004455
BT Mobile 08000 322111 08000 322111
