Can I switch from AT&T to another carrier on iPhone 15?

Tap Settings > General > About and scroll down to Carrier Lock.

If you have been using AT&T, chances are very very good that the iPhone is SIM Locked, so you will need to contact AT&T to work through the process of unlocking the phone, assuming that you have met the requirements for unlocking.

AT&T will be less than thrilled to help you with this, since they know that they will be losing a customer, so you may need to be firm with your request.

Once you have confirmed that the phone is displaying No SIM Restrictions, you can contact another carrier of your choice to get service set up with the new company.

Start by checking Settings > General > About > Carrier Lock.

If you see "No SIM Restrictions", you may likely be able to switch - depending on the country/region, carrier, etc.