@helen2023
Critical part to US roaming depends on T-Mobile/AT&T coverage and whether you can connect sucessfully onto either network.
If the phone is not connecting to T-Mobile/AT&T, change the Network Connection from Automatic to Manual and select and test both T-Mobile and AT&T and see which one you can connect.
Make sure VoLTE is enabled on the device setting. And if it does not work, try to change the Network to T-Mobile ONLY Preferred Network Type or Network Mode to 2G and see if the calls go through
Also, you can download VoIP app TextNow as a workaround to make calls and text if you have Wifi or if your roaming has data component and data is working
