@danfran
Critical part to US roaming depends on T-Mobile/AT&T coverage and whether you can connect successfully 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.
If your issue is with voice, make sure VoLTE is enabled on the device setting. And if it does not work, try to change the Network to T-Mobile ONLY, and change 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
