no need for tri band if u dont want to bring it to US. You can call US from any band its routed between the provider through satellite. In Indonesia only 900 and 1800Mhz bands are used.
HPs(cellphones) have comparable prices to europe, at least in surabaya. But u can get them really cheap on the black market(grey imports from dubai)
i think there is an island near singapure quite famous for electronics stuff but i forgot the name. maybe someone else here can help.
This side has some information on tri n quad band cells