To share my recent experiences, I recieved a 30 days visa on arrival (VOA) at Tehran airport on the 19th Jan. I had planned to extend it for another week when I was in Yazd a few days ago, but I was told that it was only possible to do in Tehran (for VOA only). I have also heard of people in Esfahan being told the same thing. Perhaps this is a new rule as I had not heard of this happenning before. I was able to extend it with no problems in Tehran yesterday (1st Feb) for a further 8 days, which was all I need so is what I asked for. I recommend going early (perhaps being polite and conservatively dressed might help) as I was able to get it in half an hour rather than having to wait a few days. This was of course after having spent around an hour paying the fee at the bank, having photos taken (as a woman I needed one wearing a head scarf, if you have your own bring 2 copies) and filling out the forms etc.