1) Make sure the phone is not blocked/ black listed (it works on the original network without any issue).
2) Check if a phone asks for an unlock code (SIM network unlock PIN/key) with an unsupported SIM card inserted. If it does, it can be unlocked by the code.
If a device does not ask, make sure the SIM is activated (works in another phone) and compatible with the device (GSM/CDMA technology).
NOTE: If the SIM is activated and compatible it might use a Device Unlock app from T-Mobile and Metro PCS USA. Device which has installed Device Unlock App can’t be unlocked by any unlock code. In this case there is another solution and you should contact us for more details.
Please double check what exactly you are ordering to avoid any trouble.