It'll be better if you can check your Mobile Phone Frequency Information first and then to order our phones. It's easy to explain what frequency for you use,Here I give 2 examples.

Example 1, I am from Iran with MTN Irancell, and i wanna order a HTC ONE M8. Let's first check the frequency of this phone, we can see it on the description: GSM 850/900/1800/1900MHz + WCDMA 850/2100MHz. Then we need to check the MTN Irancell Simcard Operator frequency in Iran: 2G GSM 900/1800MHz 3G 2100MHz, that means, HTC ONE M8 can make phone calls and use 3G in Iran.

Example 2, I am from Angola with Movicel, and i still wanna order a xiaomi Mi4. Let's first check the frequency of this phone, we can see it on the description: GSM 850/900/1800/1900MHz + WCDMA 850/2100MHz. Then we need to check the Movicel Simcard Operator frequency in Angola: 2G GSM 900/1800MHz 3G 900/1800MHz, that means, XiaoMi Mi4 only support using 2G Network to surfer internet when using Movicel Simcard in Angola.

The following is list for Mobile Phone Frequency Information by Country