If you mean calling a US cell phone customer who is currently roaming abroad, you are not charged any differently, the person you're calling that's roaming abroad will be charged the same as if they had made the call - if you were a T-Mobile customer roaming in France for example, the T-Mobile customer would be charged 25 cents a minute for a call - the same if it's incoming or outgoing.
If they are overseas and call you here in the US, nothing changes for you, it just comes out of your normal allowance. The best solution I've found is just using a Google Voice number and making data calls abroad with a cheap local SIM or esim.
US Mobile only offers international roaming through eSIM.
If your phone supports wifi calling (any iPhone, a ton of Android phones nowadays), your calls abroad will be charged as if they were being made in the US. It's a great option. I have used T-Mobile's wifi calling all over Europe and also in Mainland China and it worked great.