Cheapest way is to buy a mobile phone when you get there. Nokia 1600 + Local SIM costs about 100 YTL (about £40). All Turkish supplied handsets are unlocked and can be used in any country and on any network so can be sold easily. Beware of mobile phone shops as they generally rip off tourists, much better to buy from a supermarket like Kipa (Tesco), Tansas or Migros or a network store. Turkcell PAYG credit is the only one that can be used abroad. Vodafone's credit is only for use in Turkey unless you pay a hefty additional subscription for roaming.
You cannot use an unlocked UK phone in Turkey as they block UK handsets using domestic networks by IMEI number and the only way to get that changed is to supply proof of purchse to a local Turkish customs office. This 'service' costs about the same as buying a cheap local PAYG handset.
Once you have a local handset you can make pretty cheap calls to the UK. A 200 credit on Vodafone Turkey costs about £13 and with promotional allowance is good for about 75 minutes of calls back to the UK. Texts are also reasonable and cost less than 1/2 of what a UK network will charge when roaming.
Even taking into account the outlay on a phone the savings are pretty good. £53 buys 75 mins Vs Orange's £97.50 for the same number of minutes and the more you buy the more you save!! By giving friends you local number you will also save on incoming roaming charges although your contacts will need to shop around to find the best deal calling a mobile in Turkey!
Hope the above helps!
N-Joy