https://www.cnet.com/news/at-t-launches-esim-support-for-iphone-xs-and-xr-verizon-reportedly-up-next/

On a support page, AT&T touted the ease of using two numbers on a phone and described how its Dual Sim approach will work for users. You'll be able to choose which number you want to send text messages from when you're composing your message, and you can receive messages for both numbers at any time.

If you're on a call and another call comes in through the same line, you'll get a notification about it. If a call comes in through your other line, it'll go straight to voicemail. If you're using data while on a call, you'll have to use data for the number that you're using at the time.