Why is my phone say not registered on network

Hi @filmonsamuel

Check how many mobile phone service provider networks are shown in Settings > Connections > Mobile Networks > Network Operators.

If there is only 1 network provider shown and the SIM card in the phone is not from that provider then the phone can't make/receive calls. The phone is locked to the network provider shown.

You need to either use a SIM card from that service provider or get the unlock code from them to unlock the phone so that it can be used with any service provider in your location