My experience has been that every time you top-up you are contacted by text to take advantage of this and other offers that may be available. You then need to reply to the text to implement the offer.
Below is an example of one of the offers. Message received today:
Her yone rahatca konusmak ister misiniz? Sadece 8tl ye 1 ay boyunca her yone 80 dakika konusmak icin EVET yazip cevaplayin.
I understand this to mean that if I give 8 tl from my existing credit I am able to telephone any network for a period of one month providing I do not speak for more than 80 minutes.
Hopefully someone will correct me if wrong.