The Simple Choice international roaming is data and text only not voice. Voice is for most countries a flat $0.20 per minute. Now with that being said if you have a T-Mobile WiFi calling enabled phone and you are on WiFi calling you may call a US number or a US number can call you no charge. If you call a local number or a local calls you then you will be charged. Think of it this way, when you are on WiFi calling you are essentially in the US phone system so calling a US number or someone calling you (also a US) number the call never hits a foreign exchange which costs $$.
I know I didn't hit each scenario but hopefully the above answers them all.