To start with you will need to be in range of Turkcell, ie here in Turkey I think, as you need to get your password first.
After that, yes plain sailing via the internet.
Well I tried yesterday to top up and after receiving the password I went online to enter details, only to be told my password had expired. I tried to renew my password then, but it didn't respond until I was out in town. When I got to my computer again I entered the new password and guess what....."your password has expired"...LOL
Anyone else tried this?