The most common issues in the process of phone unlocking for Samsung, HTC, Motorola....
1) NO PROMPT
a) Phone is blocked by the network provider. Check the phone works with the original network SIM.
b) The SIM is not active. Check the SIM works in another device.
c) The phone uses Device Unlock app. T-Mobile and MetroPCS sell their devices from 2015 with a preinstalled application called “Device Unlock App.” The device with Device Unlock App it can’t be unlocked by any code. The special service has to be used.
d) Phone is already unlocked.
2) NOT REGISTERED ON NETWORK
Make sure the SIM is active and compatible with the device. If it’s possible try another SIM.
This message may occur because of the firmware or software or APN is not the latest version. Update usually solve this.
It’s possible to select the network manually in Settings select following:
1) Wireless & Networks
2) Mobile Networks
3) Network Operators
4) Select the proper network.
3) PHONE IS ROOTED
If your phone is rooted, you will need to reinstall a network authorised version of Android (a stock ROM) before you can unlock your phone.
If you do not know if your phone is rooted, easiest is to use an app - Root Checker
Once a phone is unlocked it can be rooted again.