If you have a decent WiFi connection then texting and internet should work normally.
Calling will be dependent on your cellular carriers settings for your account. A cellular signal will work normally if your current carrier has roaming at your location, or if you have added a local sim service to your phone. Your phone needs to have the bands that match the local sim. For instance, the iPhone 6 you mentioned will not work on all the common bands in China. It will be able to make calls & text, but only do slow speed internet.
You didn't mention how long your typical trip lasts. If it's just a few days, use your home service roaming. If a week or longer, consider buying sim service at your destionation.