most SIMs have accounts that allow int'l calls. I don't know why, but some accounts are domestic only... and they're tied to the SIM.

Once your account is activated. Call and get International roaming added. Mine didn't work in Cambodia, but does in Canada, US, Singapore, HKG. Maybe I didn't specify.

As for the Unlocking, if you can wait until you're in BKK. It's fast, easy, and affordable to get your phone unlocked. MBK 4th floor... one of many places to get it done.