The network type (GSM 900/1800/1900/Others) your handset supports must be compatible with the network type of the visiting network. You can check the network types your handset supports from https://www.gsmarena.com.
GP International Roaming Partners (GSM).
Most of the GP's Roaming Partner operators are GSM 900 or 1800. But there are some exceptions such as GSM 850 or 1900 network in North American countries, iden for Nextel, USA, 3G in Japan etc. If your current handset doesn't support this type of network, you can buy or rent (from respective airport) appropriate handset for using in these operators' networks