Step 1: set up a Google Voice number. This is free and available with any Google account. In fact, I have a number for both my personal email and my school email. Note - do not try to use the Google Voice app while abroad. This uses your cellular data. What I'm about to tell you does not.
Step 2: Download the Talkatone app for your iPhone. I believe they have an Android version as well. Follow their setup directions. Note: it may be a good idea to do their setup instructions just before/after you leave as it could affect your ability to use voice and video chat through your Google email. It won't reduce your ability, but it might send all of your chats and video/voice calls that normally come to your computer to your phone.
Step 3: As soon as you leave the country, turn your phone to airplane mode and then turn wifi on. Airplane mode will turn all of your data off. Be sure to disable your cellular data first. While we were in the UK, occasionally, (about every other day) our phones would magically come out of airplane mode. This happened to all four of us at least once.
Step 4: Use the TalkaTone app as you would a phone for texting and calls. You must be on wifi. If you aren't going to be somewhere with wifi, consider hiring (renting) a wifi hotspot. The best option for wifi in the UK is Adam wireless. For just 9£ per day (about $14) you can have unlimited wireless. **They do charge a 100£ deposit, but they will then subtract your usage days from that deposit and refund the rest once you have returned it.** Couple this device with an external battery pack (the hotspots are limited to about 5 hours of battery) and you have phone/text access 24/7. TEP is an another option, which is cheaper, but you are limited (even on the "unlimited plan") to 400 MB x the days rented for the total amount of data used. I have no idea how much data we used, but as my family likes to Instagram and use social media regularly, that was a bonus for each of us to have a hotspot and be able to connect if needed through talkatone/google voice when we weren't together.
Note: Adam Wireless only works in the UK, so if you are traveling to Europe, you should look into TEP. They do have plans that work (for different prices) depending on where you are traveling. The main thing is to manage your data. You can't just sit around and stream movies or video chat all the time.
I hope this helps you in your future travels!
Please note: while I did use the Adam Wireless devices on my trip to the UK, I was not paid to endorse this product. Their customer service was top-notch and they kept in touch via email and phone before and during my trip. I thought this product was worth talking about here, but I was not compensated in any way for writing this post.