Can I keep my iPhone and number when leaving someone else’s phone plan?

You can usually identify the carrier under Settings > General > About, where it may show the network and whether the phone is carrier-locked. You can also try 611, although that may connect to the current carrier. A locked iPhone may need to be unlocked by the original carrier before another provider will accept it; eligibility depends on the carrier and account status.

The person who controls the cellular account may be able to suspend the line, and the phone could potentially be reported as lost or stolen, which might block cellular service. Wi-Fi could still work, but don’t rely on the phone remaining usable. If possible, arrange a separate phone or backup contact method and contact a domestic-violence advocate or local emergency service from a safe device for help making a technology-safety plan.

Changing the SIM normally won’t erase the phone’s locally stored data. Your photos and files are tied to the iPhone and your Apple Account, not usually to the SIM. Before making changes, back up the phone, make sure you know the Apple Account password, change that password from a safe device, and review trusted phone numbers and devices. Also check whether Find My iPhone and Activation Lock are connected to an account you control.

Keeping the same number is possible only if the number can be ported. You’ll generally need the current account number and a transfer PIN or other authorization, which may require access to the account owner. If that person controls the account, opening a new line with a new number may be safer and faster. Don’t cancel the old line before a successful port, because cancellation can make the number unavailable.