@TeddieLol : It was probably sold SIM-free, meaning it would lock to the 1st SIM used in it.
To unlock a phone not bought direct from EE you need to call Customer Services on +44 800 956 6000 (option 2, option 4) to put you in touch with the Unlocking Team to request an Unlock Code. You will be asked for your email addy. They will take payment (of £8.99) from any valid UK bank card.
Once the process has been started you will receive an email from the Unlocking Team to process the unlock. It can take up to 10 days to complete the unlock.
@TeddieLol Years ago the phone would lock to the first network that the sim was from as unlocked devices were not available for a few months after release of the device. You’ll need to request the unlock of the device. Click the green wording below to see how to request this.
just to add before you say EE shouldn’t do this they haven’t as the device lock is controlled by Apple and your device would lock to any network that the first sim is from.