Yes you have to register by the 13/12 . Thats if you have a Turkcell sim card. Then Turkcell will automatically take 50 kontors off of your credit as the 5TL payment. This of course only happens if you've registered your IMEI number with them. Apparently if your on holiday with your mobile you can of course use it to phone the UK, BUT if you are fortunate enough to live here then you have to register or lose the number and all the credits left on the phone. On going to a Turkcell shop on the 12th and seeing a queue of some 50 turks or more ( and not one UK citizen) I made drastic enquires. Hope this helps