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The conflicting information is due to visa-free entries frequently being called visas on arrival.
You can enter without a visa for 30 days, which cannot be extended, but you can also buy a visa on arrival (also 30 days) for USD 35, which can be extended once for another 30 days.
So make sure to go to the visa desk at the airport and buy a visa before clearing immigration, and present your passport open at the visa page (not the main ID page) to make it clear that you have a visa that you wish to use