Is Zain Telecommunications company's offer "Your call cost at the weekend will be doubled in the middle of the week" permissible in Sharia?

There is no harm in Zain offering a deal where the cost of calls is doubled and weekend usage is given as free credit for consumption during weekdays. This is considered among the free prizes given by companies, and it is permissible provided that the beneficiary does not pay money in exchange for these prizes.