You want to change the SIM of your BlackBerry (use another carrier) and you don't even know if your phone is locked to the actual carrier, proceed as follow:
- First turn off all your Wireless connections
- Go to Options > Advanced Options > SIM card
- Press the menu button to access the Keyboad
- Enter the following code (Case-sensitive):"MEPD"
- A list of items shall be displayed, scroll to the Network section
- The status of your phone shall be displayed