The United States has two (2) different types of cell phone technologies, CDMA and GSM.
The rest of the world uses GSM. Verizon and Sprint are CDMA networks while AT&T and T-Mobile are GSM networks.
Most of the time Verizon phones will not be compatible overseas, because they use CDMA technology and it isn’t compatible with their GSM networks.
In some cases the phone will be compatible as some newer phones are made to be compatible with GSM and CDMA networks.
Take a look at our Guide to Bring Your Own Devices to get more information on CDMA and GSM technologies.
If your phone isn’t GSM compatible your phone will not be compatible with networks overseas and you would be advised to get a new phone and service there.