A friend called me asking how to use her iPhone 6 while on a trip overseas. Having just returned from a trip for abou two weeks and spending a month in Venice last year, I have some personal experience. But also found an article in MacLife [July 12] magazine on the topic of taking an iPhone overseas. There are a variety of ways to deal with this issue, depending on where you are going and how long you will be there.
1. The first thing to remember is that wifi is almost everywhere [and if there's a McDonalds or Starbucks in the area, those usually supply free wifi]. You would want a VPN to protect yourself from hackers and security issues. But, using wifi should not be an issue in most places overseas. That will help limit the need for data access.
2. However, if you are on the road and need to make a call or use your GPS or search for something online, you will need a data and call plan of some type.
3. For shorter trips, you can work with your carrier [i.e., Verizon, ATT] to get a data and call plan for a flat amount that would provide you with the ability to make and receive calls or texts, etc. I've found also that if you run over the plan by mistake, or through not turning off roaming etc., your carrier will work with you to have you buy a plan and have it retroactively apply [which can be a great savings].
4. For longer trips or if you think you'll be using data, text and calling more than on a limited basis, you can and should get a prepaid [rechargeable] SIM card from a carrier covering where you are visiting. If you'll be in Europe, there are carriers in each country which will cover you wherever you travel in Europe. To do this you will need your phone unlocked, but as I found out recently you can go into the cell store [in my case it was Orange in France] and they'll be able to unlock it for you and then sell you a SIM.
The main caution I have is before you leave the US, be sure you turn off your Data, Cell, and Roaming - or turn on your Airplane mode. And don't turn anything on unless you are on WiFi or have a good plan. Because, not only would calls come in and cost you, but data as well [such as apps updating].