If you have lost your mobile, have a look here
http://www.immobilise.com/how_does_it_work_iframe.html .Though be aware this only works if the IMEI has not been changed and most handset IMEI can be altered quite easily.
http://www.immobilise.co.uk
Provider contacts to block a handset or find out if an IMEI is blocked , ( tip of the day-- Orange are central to this so a call to Orange should quickly be passed to all providers even if you are not with them

).
Orange
Pay as you go: 07973 100451
Contract: 07973 100150
Vodafone
Pay as you go: 08700776655
Contract: 07836 191191
O2
Pay as you go: 08705 678678
Contract: 08705 214000
T-Mobile
0845 4125000
Virgin
0845 6000789
Due to the possibility of fraud the AV Forums will not allow discussion on how to unlock locked or blocked mobile phones.
If you find a mobile phone our advice is to hand it in to your local police station.
If you have a problem with your own locked phone please speak to your service provider.
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