When using your iPhone, your carrier should usually appear in the upper left hand corner.
However, if you don't have service, or if your iPhone doesn't have a SIM card, you won't be able to see this detail. However, even without a SIM card or network connection, you can easily find the carrier information associated with your iPhone in the "General" section of your Settings app.
This section of Settings is also where you'll find your phone's IMEI number and can manually update your carrier settings to improve cellular connection.
Here's how to find out your carrier.
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How to find out your iPhone carrier without a SIM card
1. Open the Settings app on your iPhone.
2. Scroll down to tap "General" in the third set of tabs.
4. First, you may be able to see the carrier next to "Network."
5. However, if you don't have service, you'll have to scroll down farther on the page.
6. In the third section of the list, you'll see "Carrier" with your carrier listed to the right. If you tap it, this information will change, but if you click again "Carrier" will reappear.