This is all based on if you are on a T-Mobile One or grandfathered Simple Choice Plan that is "post-paid" (i.e. credit check)
Check you country using the link below for any specific details and program in the number below as it is the T-Mobile Customer service number when roaming international.
Call while roaming internationally: +1-505-998-3793
For your other questions yes you must turn on roaming.
As for airplane mode, no if you turn that on it will turn off the cell radio. Just know in most countries calling is a flat $0.20 which compared to the other carriers is dirt cheap. If you have access to wi-fi like at a hotel or restaurant and you use Wi-Fi calling to a US number or receive a call from a US number those calls are free. IPhones are don't have quite the settings the Androids do with Wi-Fi calling so you have to be a little more careful but if you have Android most allow you to select Wi-Fi calling only, but only use that when you want to use Wi-Fi calling as on some phones will allow text messages to only work on Wi-Fi also.