As of sometime in October certain area codes in the US will have to use the area code to make local calls. So 123-456-7890 not all area codes will need to do it though.

It already started a few years ago in CNY

It’s going to be a new thing for us.

is this new for landlines? we've only had cell phones for over 10 years and I know it has been a thing on them for ages.

FCC 988 Crisis Line & Mandatory 10-Digit Dialing Imp...

This is what I was sent by T-Mobile. I never had to put in the area code with my cell phone for a local call.

oh weird, I'm in Michigan and my area code wasn't listed.

Yeah I’m guessing some areas are out of date or something and those are the ones listed. I dunno! Mine is on the list