Well, signing a deal doesn´t in any way mean, they actually got those missiles already via Belarus or Azerbaijan. It might as well be questioned, if the S-300 would seriously increase their defense capability or could turn into a trojan horse: As it is generally accepted that Israel views those missiles as a strategic game changer there would be less fallout from the IDF taking them out than there would be from an allout air-strike. Iran would have to respond though in a way that could easily trigger an open war - leading to the destruction of its nuclear arms capacity. With Iran at least in great part responsible for the escalation this would be one of the smoother "armed conflict entry scenarios".
In three days, Greek coastguard rescues 2,500 migrants (Reuters)
from the article: Iran official: We have obtained the S-300 missile system