I would have said avea without a doubt although its hard to get a sim in high season tourists snap them up
but Telsim has been taken over by vodaphone so may be the best to use now
amoungst uk companies the cheapest is virgin by about 5 p a min
if you going to a larger resort look out for the internet phone stalls , make sure its internet phone and not just a metered land line , they are dirt cheap to use works out about TL 2 for 25 minute call to uk basically its a meterd skype phone
dont use the public phones for international calls costs a fortune
also i would say make sure your phone is unlocked and use a Turkish sim any of the mobile phone shops have to tell you the correct prices of all companies (it changes often ) as uk companies charge a fortune for incoming calls and texts Turkish ones dont and all the turkish networks cheaper than using uk ones
another wee tip
if you have a good mobile say 12 to 18 months old that you dont use take it with you Turkish phones are around a year behind the uk so you can sell what is a old phone here for a good price there
i will find out what is best to use both price and coverage wise this year though and get back to you
where are you going the resort or region will do
and can i hide in luggage
edit
i almost forgot if you buying a simcard or mobile phone in Turkey you will need ID and as a non Turkish resident you will need your passport although most shops wont bother checking it its just one of our funny laws
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