I'm also with O2 pay and go and paid for the international bolt on - it was £5, a one off payment with no expiry, which means I can text a Turkish mobile while I'm in the UK for just 10p per text - it was well worth it (check the internationall caller section on the o2 website for details).
There is no charge to the recipient if you are calling a Turkish mobile while you are in the UK.
Your Turkish friend will only be charged to receive a calls if you call using your UK mobile whilst you are in Turkey. Don't think they are charged for a text sent while you are in Turkey.
I find it works out cheaper to call Turkey from the UK on a mobile or land line using a Post Office international phone card. You can buy them in denominations of £5, £10, £15 etc, think the current call rate to a Turkish mobile is 16p a minute a little less to a landline. They sometimes have special weekends where you can call various countries free of charge - information is on the Post Office website.
Also (bizarrely) I've discovered with O2 it's chepaer to use MMS for texting the UK - MMS is only 25p, standard texts are double! And of course you can add a photo to a MMS to make all your friends at work jealous!!
Hope that helps!
Mazz
