Every mobile device has a unique serial number known as an IMEI (International Mobile Equipment Identifier). If your handset was lost or stolen giffgaff would use this number (IMEI) to block your device & prevent anyone from using your device on any other network, this process is called IMEI blacklisting.
Everytime you switch on your device, insert a new SIM or go to make a call your IMEI number is checked against the CEIR (Central Equipment Identity Register) database also known as the IMEI database. If your device is listed within the CEIR blacklist, then giffgaff (and all other networks) will refuse to send a signal to your device (so you will see emergency calls only) or in some cases you may receive a signal but be barred from making / receiving calls and texts.
If your devices IMEI number is listed on the CEIR (Central Equipment Identity Register) blacklist database your device is unusable.
This is where it becomes difficult as all devices react differently, some devices display messages such as "SIM not accepted" if they have been blacklisted making it really hard to tell the difference between locked and blacklisted handsets. Others just don't switch on or behave in other ways which is dependent on the make/model of handset.
For example, an iPhone that is blacklisted will receive a normal signal, the internet will work normally but you will be barred from making / receiving calls and texts.
The only definitive way to check if your particular device is blacklisted is by visiting Checkmend - there, for a small fee (£1.99), you can enter the IMEI number of the device (you can obtain your IMEI number by simply dialing *#06# from your device) and receive a report that will tell you if the phone is blacklisted or not.
The Checkmend service is independent of giffgaff. giffgaff does not receive any share of the fees paid to Checkmend.
Please see the below sample report from Checkmend:
Only the original legal owner of the device & the reporting network / agency would be able to have the blacklisting status removed from the device.
If you purchased the handset via ebay or gumtree then it would be advisable to contact the seller & the website owners. You should be able to get a refund / chargeback if you used a credit/debit card or paypal to purchase the device. Please ask our community for assistance.
Please note: giffgaff cannot remove a device from the blacklisting database unless you are the member who reported the device lost or stolen via your giffgaff account & giffgaff were the network that placed the devices IMEI number on the CEIR (Central Equipment Identity Register) blacklisting database.