If you buy a Sim Card in Turkey as we do all the time you need to use it. If you are away for 6 months or more it 'dies' and you need to buy a new one. Also, you need to use an unlocked mobile phone to use this (a bit obvious I know). Unlocked mobiles will also not work after 6 months so you need to change phones with the Sim card every 6 months. We have three kids so this is never a problem. The calls are much cheaper (last Summer £20 top up lasted nearly 6 weeks and there were a lot of calls to the UK over this period as a member of family had to go to hospital). Be careful where you buy the Top Up cards as in some of the more touristy areas the local shopkeepers have a habit of adding on a few lira on top.