How did I link my iMessage to my new phone

To activate iMessage and FaceTime using your Phone number

First, check to know whether you have an SMS plan enabled at least for One SMS*** with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS*** Only)

If you can't turn on or sign in to iMessage or FaceTime - Apple ...

*** You may not have an SMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS services.

These instructions could be helpful:--> Set up Messages on iPhone – Apple Support (UK) , and/or:--> Add or remove your phone number in Messages or FaceTime - Apple Support

Regards

Giulio