Iphone is still locked to network after restoring phone?

Last year I bought an iphone 6s from carphone warehouse and decided to put a 3 sim card, but it ended up locking the phone to the 3 network. Now I would like to move to O2.
After phoning customer service and the 3 website, it made me believe that I can unlock it by backing up the phone then restoring it will unlock.
But after backing the phone with the 3 sim card and restoring the phone with no sim card inside, the phone is still network locked.
Please leave your experiences and solutions

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