That phone would be pretty widely usable for calling / texting. Data is where things get dicier.
That curve supports:
GSM 850 / 900 / 1800 / 1900
3G Network HSDPA 850 / 1900 / 2100
HSDPA 900 / 1700 / 2100
According to this chart, it should handle at minimum 2G in every country listed
http://www.worldtimezone.com/gsm.html