Long time TMO user here. I travel with iPhone for years, most recently with 5s. Have used older 3 and 4s models extensively in Bkk and never a problem with roaming charges.

Re Canada calls, can you access your usage via internet and check call log?

In Maldives, merely have 5s on airplane mode with the wifi turned on. That way your 'phone' is off but you can still access email, internet, other apps via wifi.

If you're super paranoid, go to settings, cellular and turn off data, LTE and Roaming.