Use SIM card on international roaming only for voice calls (sans mobile data)
Yes, it is possible to use your SIM card with international roaming without using any data. Just make sure the Mobile Data is turned off under Settings app → Mobile Data on your iPhone 6.

 

Additionally, under Settings app → Mobile Data → Mobile Data Options, you can choose to turn off Data Roaming. This is a smart settings which will prevent data usage while roaming. (You may have your reservations regarding this setting, but turning off Mobile Data gives complete peace of mind).

As long as the Mobile Data is turned off, you’ll be fine. No app will do any data transfer/use Internet this way (as long as Wi-Fi is also turned off).