For my latest trip to Europe I wasn't sure what to use for wireless service other than one of the relatively expensive regular plans from one of the top 4 carriers. It might be worth it to subscribe for just one month, but I thought there must be a cheaper way. This is what I did, instead:
I installed Google Hangouts in my GSM smart phone (including the Hangouts Dialer) and connected Hangouts and my voice mail to my Google Voice number. (You can also forward your own number to Google Voice) Upon arrival in Europe I purchased a local SIM at an airport newsstand. The one I chose was Lebara Mobile which cost $3US, with the lowest topup at $23US. Check their leaflet to choose a plan and text the plan name to a short code number.
Once my phone acquired a 4G signal, I was receiving incoming calls to my US number and voice mail notifications through Hangouts on the broadband connection. I was able to make local calls through the regular dialer at about a penny per minute. Calls to the US completed through Hangouts were free.
The only challenge was where to the buy the SIM. There are "SIM" stores that charge about twice as much as other sources. Instead, you can buy a local SIM from almost any airport newsstand and even from vending machines for much less. SIM's can be purchased in the US, but then they must be shipped. I can't think that far ahead. This was easier and cheaper than I thought possible. I received free calls to my normal US number, I called the US for free, I called UK numbers for about .02USD/min. and had 4G broadband access everywhere. What a trip!