I accidentally Sim Locked my new iPhone 7.

HeelBoiiBm wrote:

The SIM card which came with my phone on contract wasn’t appearing/working during setup, so I used the old SIM card from my old phone to just check it was working. It then let me past the setup but what I didn’t realise until after is that it locked my phone to O2 network and not BT which I am paying a contract for. Anybody able to help on how I could fix this, and I’m not too bothered about breaking my warranty as long as it fixes it so I can use my new plan with my phone. Thanks in advance. I am using IOS 11 I believe as it is the most recent update.

You contact O2, only the carrier that the phone is locked to is authorized to unlock it.

No. That's not how unlocking works on an iPhone.

How to unlock your iPhone for use with a different carrier - Apple Support

No matter what SIM you put into the phone it will not lock it! The phone is either carrier locked (locked by a specific carrier) or unlocked. Probably your normal sim didnt work from the begining in the phone because the phone was already carrier locked by O2.