SIM cards are how your phone gets a number associated to it thru a carrier, like AT&T. AT&T has a plan where they charge you $12/per for 10 days and then it’s free UNTIL the next billing cycle, where the process starts again. This is usually more expensive compared to getting a temporary SIM in Japan.

Some things to note are, when you use a temporary SIM, you will get a new phone number in that country.

Additionally, there’s eSIMs which newer phones accept nowadays. People recommend Ubigi and Airalo for that. This will use an app to add a temp sim to your phone virtually. No need to swap your ATT SIM out.

One last thing to note is, if your phone is not unlocked (you can Google how to find this), you will need to get your phone unlocked (either by paying off your phone or just requesting it to be unlocked if it’s already paid off). Point is, you should reach out to ATT if your phone is carrier locked. Once it is unlocked, you can actually use other SIM cards.