This is a question everyone asks when you buy a Smartphone or cell phone of any brand. If you are a person who likes to travel or have to do it frequently for business you should know how to choose the device you are going to buy to take advantage of it as much as possible. If you want to use it worldwide you should consider using technology (GSM) before buying. (CDMA) devices are mostly used in U.S.A and will be limited for use worldwide.
Meaning a Smartphone with GSM technology?
It is the most common technology worldwide and operates in 4 frequencies (850/900/1800/1900 MHz):
GSM 900MHz & 1800MHz GSM (used in Europe, Asia, Australia, Middle East and Africa)
GSM 850MHz & 1900MHz GSM (used in USA, Canada, Mexico and most countries of Latin America)
As you can see from there comes the famous term "Open Bands" a process that can be done online through sites like www.unlock-direct.com . Once you have unlocked your phone it can be used in any of the 4 bands mentioned or virtually anywhere in the world !!
Meaning a smartphone with CDMA technology?
The CDMA technology is less common but is used in USA by carriers like Verizon and Sprint which means that if your phone only supports the CDMA technology you would not be able to use it anywhere in the world as will not be supported.
In conclusion you better make sure you buy a smartphone with GSM technology and enabled for 3G and / or 4G which will give better performance for data transmission.
All this is very easy to check and you can read the specifications on the box your phone before buying it.
Check the bands in your country or the country you travel in: http://tendencias-celular.com/blog/2015/9/8/lista-de-paises-con-tecnologia-gsm-y-sus-frecuencias