how do I know if my iPhone 6s is locked to a carrier?

iPhone's sold from Walmart will generally lock to the first carrier used to activate them. As such, in your case its safe to say its locked to Straight Talk.

You can test this by inserting a Sim card from a different carrier into the phone. If it issues an error such as the one below, then the iPhone is locked:

If it issues no error, then its not locked.

You can request Straight Talk unlock it if you wish, as per their Unlocking Policy here (Section VII):

https://www.straighttalk.com/wps/portal/home/h/legal/terms-and-conditions

You can follow the steps here, one of which is to contact the carrier it is locked to:

How to unlock your iPhone for use with a different carrier - Apple Support

No. It is locked to StraightTalk. It can ONLY be used on StraightTalk unless you get it unlocked by StraightTalk.

See this article: How to unlock your iPhone for use with a different carrier - Apple Support

You can make an appointment with an apple store and they can tell you if it is locked with a carrier. You can also contact your carrier to see if it is locked with their service. Keep in mind that there are two types of phones that were made because the carriers require different antennas on the devices to work with their service. So Sprint and Verizon networks are CDMA while ATT and T-mobile are GSM. You have straight talk so it would work with any ATT or T-mobile service but not with any Verizon or Sprint service after you figure out if it is locked to your carrier.

chellester wrote:

Keep in mind that there are two types of phones that were made because the carriers require different antennas on the devices to work with their service.

While true for newer and much older models, iPhones from the 4s and up until the 7, had both types of Antennae built in to all models so can work interchangeably on both types of carriers. It's just that most CDMA carriers will most often not activate a phone on their network that is not in their database of devices, and iPhones not intended for them are in most cases not added to the database during the manufacturing process.

i tried putting in my activated verizon sim card and it didn’t have a message pop up saying that it doesn’t work with the phone. it showed the service bars but not “Verizon”

That generally means the phone is unlocked, however, Verizon would need to add it to their CDMA service for it to work entirely.

If the "Sim Not Supported" error was not shown its unlocked. It won't simply show no service, if the phone is locked and the sim card is not from the carrier it is locked to. It will issue the error in all cases.

You can try a sim card from a GSM carrier such as T-Mobile to verify this.

Nope. iPhone 4s, iPhone 5, iPhone 5s, iPhone 6, and iPhone 6s all have both types of hardware inside.

Activation again is dependant on the carrier. Verizon can choose not to activate it with their own service, that does not mean the CDMA hardware does not exist inside.

Walmart does not sell unlocked iPhones. When you activated it on StraightTalk, it locked to StraightTalk. Only StraightTalk can authorize unlocking it.

That's because the phone is locked to StraightTalk.

Sorry, no, its not locked to Straight Talk.

If the error message was not shown, it is not locked. It's just that Verizon requires extra setup for the iPhone to work.

Try it with a T-Mobile sim card.

Actually this is false. I got my iPhone 7 from ATT and tried giving it to a friend who had Verizon and it would not activate. It is only iPhone 6 that has both in it.