The Turkcell cards expire after 6 months, we had to get a new sim card & number for ours this year as it was 7 months between visits.
I think you lose your credits after 3 months BUT have yet to find that out for definate. I think the only way to keep it is to leave it with someone or get someone to take it out with them & top up.
Ours went on a phone on the Orange tariff but when you get back home with the turkcell card in it comes up as no network found.
Heyjo - A few months back it cost 4 units per minute to phone the UK via Turkcell. A unit then cost approx. 4p so approx. 16p a minute. I know texts are less expensive. It was costing us about the same for a call a few doors down the road in Turkey as it was to phone home.
If you want cheaper calls & use the phone a lot I believe the more expensive phone cards give you more time plus Turkcell kept sending us messages which translated meant the more you spend the more units they give you free.
Linda