Skype or We Chat or Face Time or Text ? T-Mobile or French local Xim card.

This is pretty simple. Our experience is limited to Europe but when time isn't critical and internet is available skype and face time for us work very well. If time is critical, then you will need a local sim card for your phone.

FM,

Using Skype, FaceTime, Viber or whatever should work reasonably well when you have Wi-Fi service available, but if you're out touring you'll be limited to cellular data. That can be expensive especially for video calls and this is definitely something you'll need to sort out prior to your trip.

The least expensive option for keeping in touch is text / SMS as received texts are often free. Voice calls can be very reasonable if used sparingly. Which option to choose will depend on the specifics of your situation. It would help to know which cell network you use at home? They may offer an inexpensive travel roaming option.

If you need to be accessible for "emergency calls", the least complicated method is to simply use your home cell plan. Callers simply dial your usual number and the network will find you wherever you are. However, you'll be paying for each incoming call, usually billed in one minute increments. With a roaming plan, the costs can be reasonable.

You won't be able to use a local SIM card unless your phone is "unlocked".

You might find this thread helpful. https://community.ricksteves.com/travel-forum/tech-tips/my-experience-using-t-mobile-s-simple-choice-plan-in-6-european-countries I also had a very positive experience last year in Italy with our T Mobile Simple Choice plan. Almost to good to be true, but it is.