I believe that iPhone 6s comes unlocked, but you can check with your carrier about that. The easiest thing to do would be to stop at the nearest cellphone store or kiosk in the airport and pick up a SIM there. A local SIM will have the absolutely best prices on calls and data, much better than any roaming or international plan from a US carrier. The store can make sure it is working before you leave. Since you are planning to use it in more than one country, make sure that whatever SIM you buy can have minutes added to it easily online or in a local location. (You could also pick up a different SIM in each country to avoid the slight upcharge for roaming, but it's probably not worth the trouble.) I'm not sure what James E is saying about standing in line, because when I was last in Spain, I walked into an Orange store, bought a SIM and had them set it up, all in less than 10 minutes, no waiting in line at all.
I have never used Telestial, but I do carry a Eurobuzz SIM in my two-SIM phone. It is a postpaid system - no fees at all unless you make calls. Your number is permanent, so you can put it in a drawer between trips, and as long as you update your credit card information, you're good to go. The downside is that it isn't the cheapest to use. I keep it as an emergency backup - it's the number my family has at home to reach me on in an emergency. I use a local SIM in the other slot because it's cheaper to use. Most recently, my Orange SIM cost 9 euros, and had 9 euros credit on it. I called home three times, probably 15 minutes each call, and used it a tiny amount for calls when I was there. I never used up the credit. Of course, I wasn't using data, either.