Why is my phone locked

[white_junWJ]

Traveling to Japan and found out that my phone is a locked one, can't add other services besides AT&T, but I bought it straight from Samsung so why is that??

[meself]

The phone will generally lock to the Sim card network when activated

[ChaosSaber]

Mine is unlocked and never locked to a carrier

[LongHiker]

@meself "The phone will generally lock to the Sim card network when activated"

This is absolutely not true. 

[meself]

Very true a locked phone to network and a unlocked phone will also lock to the first Sim card network also applies to another company source actual users months to years reported

[LongHiker]

@meself I buy an unlocked phone from Samsung and have done it for years. I insert my AT&T SIM card. The phone does not become locked to AT&T. I can switch the SIM card to another carrier at any time OR install a eSIM from another carrier. 

[realaud]

No, inserting a carrier's SIM into an unlocked phone DOES NOT lock it to the carrier, but it does identify it to the carrier so that the carrier can provide updates.  One is free at any time to switch carriers, add an additional SIM, remove the SIM and replace it with a "travel" SIM and generally do whatever you want with it, without the carrier restricting your actions and which settings you can utilize or turn off.

[tootieF]

Apn settings

[tootieF]

Hiring networks

[ChaosSaber]

Check your order history and see if you bought a AT&T phone, if you did, it will be locked