It is recommended that you buy a local prepaid sim card for US$3 on arrival from any Cellphone store (there is one in Timor Plaza on way into town from the airport). Local prepaid SIM cards can also be purchased from street peddlers but at a higher price. Once the prepaid sim card is activated, you can top up(補給する、満たす)the credit by purchasing "Pulsa" top-up strips from street peddlers.
(http://wikitravel.org/en/East_Timor)