Why am I getting a message that says I am roaming when I place a call?

[SarcasticDragon]

I have an older phone that has starting telling me I am roaming while trying to make a call recently.  As soon as it starts trying to ring, there is an automated message that tells me I am roaming and that I need to contact my carrier to purchase something with a pin.  It mostly happens while I am at work, which is literally 1 mile from my house.  When I get home, it works just fine.  I live in Wisconsin.  I have a Galaxy S7.  I would really like my phone to continue to work.  What is going on?  This just started withing the last two weeks.  Prior to that, I was able to make calls from the office or near the front door at work.  The main part of the building doesn't allow my phone to work since it is a steel building.