While Iran has been unable to get modern Russian S300 surface-to-air missiles (yet), they have gone ahead and built a nationwide air defense network using Chinese HQ2 missiles. These were first purchased in the 1980s, but since then, Iran progressed to the point where they are now building HQ2 missiles and radar stations under license. It's believed that China has quietly allowed Iran to build the latest versions of the HQ2 as well. These are not quite as effective as the S300, but are pretty formidable.
The HQ-2 is a Chinese version of the Russian SA-2 system (from the 1950s). The Chinese have upgraded the SA-2 with modern electronics, an improved warhead, better rocket motors and more maneuverability. Their inventory of missiles is believed to be about 10,000. However, many are older models, and many of these are probably of uncertain reliability. American electronic countermeasures can probably defeat all models of the HQ-2. Newer models of the HQ-2 have a range of 40 kilometers, and will hit the target 70 percent of the time (if there are no countermeasures.) The HQ-2 radars have a hard time dealing with stealthy aircraft, and the radar is needed to guide the missile to its target (via radio signals from the ground to the missile).
China has been able to purchase Russian S300 systems, but still has an incentive to improve their HQ2s. That's because about half of their 150 air defense missile battalions are equipped with HQ2s, and that is not likely to change any time soon. So the Chinese are probably continuing to make improvements in the HQ2, and selling the technology to Iran. China wants long term access to Iranian oil, and helping upgrade Iranian air defenses is one way to help with the oil situation.