Many years ago, you may have purchased a cell phone, and after many years of using it, you want to use that phone with another phone service. You take the cell phone to the new phone service, but they tell you that you can’t get service with them using that old phone. There is a good reason for that.
This has to do with two systems used in older cell phones. They are the CDMA and the GSM. CDMA stands for the CodeDivision Multiple Access, and the GSM stands for the Global System for Mobiles. The global system for mobile has a keyword in it, which is universal. Therefore, it is better to use that for general usage.