I took a brand new unlocked Tri-Band phone to pakistan (Sony Ericson W350a) and it didn’t work. It had 900/1800/1900 frequencies I believe. It works here with T-Mobile and AT&T here in the U.S.
I have taken unlocked phones with me many times for my folks back home. They have all worked but they were all quad-bands too.
I guess I learned my lesson but which frequencies are being used in Karachi Pakistan, so I know better next time.