(Sorry for writing in English - my written Dutch is hard work nowadays!)
It seems people of many nationalities (including Dutch and Belgian) would be able to get a “Visa on arrival” at Tehran airport. Has anyone actually done this? It is very hard to find any information about this.
Absolutely not possible. You need an invitation — to get a “reference number” of granted request with invitation takes approx. 3 weeks — once you have this reference number, the Embassy in your country can give you the visa swiftly.
I just did it and it worked perfectly fine. I didn’t even have a return ticket because I’m taking the train to Turkey.
I did not have an invitation, just a friend who lives in Iran and I gave them his phone number. They called him, asked him who I was and everything was ok, and within 15 minutes I had a 15 day visa.
Good luck with it!
The question is: would they have let you into their country, if they were not able to reach your friend? I’m sorry, but this IS an invitation!
Just don’t forget, the biggest problem with applying for a visa on arrival is not per se the immigration of the country of arrival. More and more often, difficulties are caused within the country of departure. The air companies of Schiphol Airport (or whatever airport) can refuse me to board a plane if I can’t prove that I fulfil all visa requirements. An invitation letter is just an example, but many companies want an onward ticket, i.e. a prove you leave the country. Hence, it’s a risc!