Most of the world uses GSM while few counties also supports CDMA (Korea, Japan, Brazil, and Israel).

If your phone happens to be a CDMA phone, it is most likely going to work in the US (roaming rates may still be high though).

If you have a GSM phone, you should check whether it supports the US frequency band (GSM 1900), which is different than the rest of the world, but many phones do support more than one mode.

Main reasons for renting a US phone are:

1. Many foreign phones simply don’t work in the US.
2. If your home-country cell phone does operate in the US, the roaming charges may be extremely high.
3. Many areas in the US have limited or no coverage by certain cellular companies. When you rent a US phone, you can select one that is guaranteed to work in the cities/states you plan on travelling to.