Those of you who travel quite a bit undoubtedly own more than one device due to frequency band restrictions put in place by both carriers / OEMs, as well as local regulatory agencies. It’s common to find regional variants of pretty much the same devices from Sony, LG, Samsung, and other OEMs. And of course, these companies want to turn a profit, so they often sell multiple devices rather than one that can switch between all of its hardware-supported frequency bands. Surprisingly, most phones works pretty well with frequency bands that are not officially supported. In fact, this is often just a software limitation that can be circumvented with tools available on XDA.
You can try to unblock some frequencies on your own thanks to two XDA Senior Members: fffft, who found a method working for the Samsung Galaxy S5, and vndnguyen, who developed an application to calculate the required values for Qualcomm devices. The method is very (obviously) hacky, but it’s definitely worth a shot if you want to use blocked carriers while visiting other countries.
If you want to add frequency bands, you need to prepare yourself for lots of reading. The process is not as easy as simply rooting, and requires some tools like QPST and QXDM. If you follow the thread carefully and everything goes right, you will be able to use the device in practically any country in the world. You will save some money and time required to find a device on the local market. Naturally, doing this also carries with it an inherent risk, so please make sure you read and understand everything to the letter before proceeding.