You can buy a Turksih sim card for about £10 including some credit already on it but to use it in your phone you will need to register the phone or it will lock your phone up.

I'm not sure how this is done now as when I had my Turkish sim card you didn't need to do this, I think you can do it at the airport somewhere or maybe a Turkcell shop.

It is cheaper to phone that way but I personally wouldn't get a Turkish sim just for a holiday. The best and cheapest way to phone home is to buy a phone card and use a public phone, cheap rates are in the evening.

100 credits for turkcell sim is between 16 and 18 ytl plus 10 credits bonus for your phone bank the same price for telsim/vodapone
the bigger amount of credit the better deals you get but remember it only last until the date given when you top up ussually 90 days
but the phone will stay open for 6 months before the sim is shut down
you can buy and register your sim and an english phone if it is unlocked at any turkcell shop or vodapone

I think the last time I put point to my turkish ( Turkcell ) phone and SIM card it was about around 40 liras for 250 points

http://www.turkcell.com.tr/en/plans/prePaid/bizbize

Unit card prices
25 Units * 5 TL
50 Units * 9 TL
100 Units 18 TL
150 Units 26 TL
250 Units 42 TL
500 Units 77 TL
1000 Units 140 TL

http://www.turkcell.com.tr/en/plans/prePaid/kontorprices

It is now only 4 units/0,305 TL per minute of a call made to a landline (home/office number) in many countries
http://www.turkcell.com.tr/en/international

I want to buy a Hazır Kart;

Documents needed for individuals:
ID The alternatives of this document?
Handset invoice

--------------------------------------------------------------------------------

Documents required for foreigners:
Valid passport
Handset invoice

http://www.turkcell.com.tr/en/plans/prePaid/iwanttobuyahazirkart

the best advice I can give you is if you go into any telecommuncations shop either Turkcell or Vodafone ( Telsim ) you can also buy a cheapy phone turkish cell phone for about 50-60 liras..plus the card ( turkish sim ) and calling points..or many electronic superstores as well that deal with phones..if you travel frequently to Turkey..say every year for a holiday it is worth it..if only for a weeks holidays dont bother..but for 2, 3 weeks or more might be useful

And Turkcell has a wider range over Turkey than say Avea or Vodafone..

also read here;

http://www.turkishliving.com/forums/phone-adsl-pc-turkey/23692-turkcell-charges.html

http://www.turkishliving.com/forums/phone-adsl-pc-turkey/23654-keeping-turkcell-sim-card-active.html