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

While traveling abroad, you can select network in two ways: Usually, your handset will automatically select the strongest network available in that place. While you are moving, your handset may change or switch network automatically depending on network strength of that area.

If “Network Selection” or “Operator Selection” option of your handset is set as 'Manual', you can select any one the available networks in the following way:

• Go to the "Network Selection", "Phone Setup" or "Settings" option menu (depending on your handset model)
• Press "Search" & your handset will search for the available networks (this may take few minutes)
• Available networks will be shown in the display.
• Select the network you wish to use and stay connected