Performing a SIM unlock can be a great way to switch carriers while still keeping your current phone.
Now while unlocking your phone does provide some immediate advantages, it does not mean your phone is automatically compatible with every carrier in existence. One thing we must remember when unlocking your phone to switch carriers, is what technology your phone supports.
The most common network technology on the market today is GSM (Global Systems for Mobile) and is used widely across the entire globe.
Many carriers across the United States support GSM technology and in fact, GSM is used by two of the largest carriers, AT&T and T-Mobile.
The second common network technology on the market today is CDMA (Code Division Multiple Access) and is used primarily in the United States rather than the across the globe.
Now for example, if you have a T-Mobile or AT&T phone, your phone will likely support GSM technology which means you will need to switch to a carrier that also supports GSM technology. If you attempt to take a GSM phone to a CDMA carrier, the phone will most likely not be supported.
The same rule applies to CDMA customers. If you have a Sprint or Verizon phone, your phone will likely support CDMA technology which means you will need to use a carrier who supports CDMA technology. If for example, you attempt to take a CDMA phone to a GSM carrier, the phone will likely not be supported.
There are currently some exception to these compatibility rules. We've noticed that newer phones such as iPhone 6 (or newer) and the newest Samsung Galaxy phones are compatible with both GSM and CDMA carriers. Please verify this with the carrier before attempting to switch.
To sum this all up, more often than not, the technology in your phone needs to be compatible with the technology used by the carrier. You can check here to see whats available to you depending on your phone supplier.