I was told in Alanya that my phone is automatically shut off after some weeks. I cant remember the exact range somewhere between 1 week and a month. This is to discourage theft. So I had no need to register because it works as long as I was there. For me it was invaluable as many turks are still not online.
If you are in Turkey on holiday - the best thing to do is to buy a 5 TL phone card from a corner shop - or only 4 TL from the PTT Post Office - lots of phone boxes all around - and you can talk to an English landline for about half an hour for that price.