If you are on Straight talk bring your own phone program and you wanted to use another phone and its SIM card is a different size, or perhaps you wanted to use the new AT&T LTE sim card to get LTE speeds, then you have move the service to the new SIM card of the new phone.
Note: If you want to know how to get an AT&T LTE sim card, then visit this site http://www.prepaidphonenews.com/2011/12/how-to-get-tracfone-net10-or-straight.html
There are two ways to switch your sim
Option 1 : Call the customer service and go through the maze of prompts and wait for a long time to get to the customer service and then explain go through all the process over the phone.
Option 2 : This is the easiest option.
Go to www.straighttalk.com and go to you account page.
Step 1. Under my account click on “Activate Phone or SIM Card” as shown below
Step 2. Select “Transfer my existing Straight Talk Service” as shown below
Step 3. Enter the new sim card information in to the field marked with 3 and select your current cell phone marked with 4 as shown below picture.
Step 4. Click on submit and finish the process.
Step 5. Switch of the phone.
Step 6. Replace the SIM card in the phone with the new SIM Card. Wait for 5 mins.
Step 7. Power on the phone and you should be good to go instantaneously. Your cell phone should work on new sim card.
Step 8. If your phone supports LTE on AT&T network then you should see LTE in the top bar for your AT&T LTE sim.