https://www.theverge.com/2018/12/11/18136504/how-to-esim-dual-sim-apple-iphone-xr-xs

Sometimes a single phone number isn’t enough — if you need to go abroad, for example, or need separate business and personal numbers, two is definitely better than one. Until recently, unless your phone could accommodate two SIMs, you were out of luck. However, Apple has now made it possible for owners of an iPhone XR, iPhone XS, or iPhone XS Max to add a second line of service by using an eSIM (or digital SIM), bypassing the need for a physical SIM card.

This will let you easily add a local carrier if you travel internationally. You can even have two different plans on the same phone — say, one for voice and messages and one for data. Although currently your choice of carriers in the US is a little limited: only Verizon and AT&T offer eSIM capabilities, with T-Mobile coming later this year.