If you live in USA and have a USA phone for normal use, I really think that signing up for Google Voice is a great backup plan for a trip to a foreign country.

I have planned on some trips to buy a sim card in a foreign country but it has been low enough on my priority list that I have not done so. Thus, my foreign trips have all been without a working phone plan. Then, my phone became a wifi only device.

With wifi and Google voice, I can make voice phone calls from anywhere in most countries where google functions without restrictions. I have called my credit card companies when my cards stopped working from Budapest and Reykjavik. Even within USA, where both my bike touring partner and I had non-functioning cell phones due to poor cell coverage, I could use my phone to make calls using a restaurant wifi to make lodging reservations or to cancel them.

If I had a data only sim card, I can make cell calls with google voice using the data plan. To check that this will work, I have made calls with my phone using data plan while my wifi was turned off.

The down side, is to receive a call, you phone has to be connected to the internet. So, I tell everyone that if they call me on that particular number that they should leave a message because I typically do not have my phone connected to the internet. I get an e-mail with a recording of their voice mail.

The other downside is that a lot of people assume that if you called them, that the number you called on is a good phone number to call you. I have told a lot of people to call me on a different phone number, but they forget what I told them and call my on my google number instead - but that is a problem without a solution. When I travel, I might not check e-mail for days, thus do not get their voice mail messages in a timely manner.

If you call a foreign country with Google Voice, there is a charge, you need to have some money in your account with Google to make sure that you can make calls if you plan to call to foreign countries. I wanted to make sure my phone and Google Voice were functioning when I arrived in Iceland, I asked the receptionist at the hostel if I could call her to check my phone, she said sure, so I called, she did not think it was me because it had my USA based google voice number on her caller ID, I confirmed that was my call, she answered and we had a short conversation. And later 10 cents was deducted from my account because I made a call to a foreign country.

It has worked well enough for me that getting a foreign sim card has become a low priority for me, even though every trip I plan to get one.

I use an android phone, I assume this works with an apple too but I can't say for sure. My Budapest call to my credit card company, that was when I still used a flip phone, that call was made on my small laptop (or windows based netbook).