Sophia has presented us a site with a clear explanation.
"Very often, unless can be used instead of if not. Come tomorrow if I don �t phone/ unless I phone. He �ll accept the job if the salary �s not too low/ unless the salary �s too low.
However, there are some cases where unless is impossible.
A will happen it is not stopped by B, but unless cannot be used in sentences that say A will result from B not happening. Typical mistake: I �ll be surprised unless he has an accident. The correct answer is I �ll be surprised if he doen �t have an accident. I �ll be quiet glad if she doesn �t come this evening (Unless is impossible)" (Swam, 1986: 610).
I �m referring to the first conditional but it can be applied to the second one, too, as the example shown by our schoolmate.
Swam, M (1986; Eleventh edition). Practical English Usage. Oxford: Oxford University Press.