Is turning off data roaming enough?

Garrettsctt wrote:

Just doing phone calls doesn't use data. That's the cellular service. Data is used when you surf the Internet or use an app connected to the Internet. so turning off data roaming should only effect data usage and not effect sending and receiving phone calls. 

VoLTE and VoNR do use data for calls.  Whether or not that is counted for data use when roaming.  I don't know.  iMessage needs a data connection to function properly.  Using WiFi Calling as much as possible can help a lot.  Putting the device in Airplane Mode or disabling the T-Mobile eSIM when not connected to WiFi can also help.