We found it much less expensive to buy a local cell phone in countries when we got there than to try and make our existing cell service work in different locales. If you'll be cruising a locale longer than a couple of weeks, it may actually be more cost effective to buy a local phone and the prepaid service - in several Caribbean locales if you buy on a "specials" day, you can get triplicate or even quadruplicate minutes which significantly lower your call costs. Not sure about Europe having never been there. Also - when we first started cruising, we brought along a "open" phone so we could load the local sim card and not buy a local phone. This worked to a point, but it was frustrating to make it work - talking to tech support in a foreign language is difficult! We finally found it easier to just buy the cheapest phone they had available and use it. Unfortunately, most sim cards are only good for a certain number of months before they "expire" -- so you may not be able to buy a local phone one year, leave and use it again when you return - or you may, it depends on the country. Some countries in the Western Caribbean are now literally gluing the back of the phone on so it's impossible to replace a sim card, forcing you to buy a new phone when you return a few months later.
Hope this adds another perspective to your question.