Phone network locked for no reason

Good day.

My cousin had his phone blacklisted (network locked). It is an iphone X bought in 2018, on contract. He had finished paying off the phone and in return got a new phone, which he then handed the phone to him. Can this error be rectified, he in a different country at the moment which makes things tricky because the phone was bought on his mothers name, and at the store they said that she to come to the store to unlock, which is impossible because she is in Swaziland