Text on iPhone through wifi in Japan?

My friend recently went to Korea for a couple months. She told me that she deactivated her at&t plan from the U.S. and used her iPhone's wifi to text for free via wifi to the states. (Using textPlus application)
I am hoping that if I deactivate and take my iPhone I can do the same while I am in Japan. Is this possible?

Another thought.. can I deactivate, take my phone to Japan and get a Japanese plan activated on to it? I would hope using a calling card to call my family would cost less than using a ridiculously expensive roaming plan. Right?

Thanks for the help! :)

WiFi operates completely separate from any phone plans. Wifi can work anywhere there is an open wifi connection.

I am hoping that if I deactivate and take my iPhone I can do the same while I am in Japan. Is this possible?

This should work as long as you have access to wifi. Note that free wifi is NOT widely available in Japan.

Another thought.. can I deactivate, take my phone to Japan and get a Japanese plan activated on to it?

Other posters have recently expressed difficulty doing this. Japanese carriers apparently won't sell a contract sim card alone.

I would hope using a calling card to call my family would cost less than using a ridiculously expensive roaming plan. Right?

Yes, a calling card is cheaper for international calls than pretty much every cell phone plan whether roaming, contract , or prepaid. It is even cheaper (or even free) to use skype or voip services over wifi.

Take a look at "fring" and Skype for your iPhone. You can use fring to sent text if you have wifi access. We will be in Japan and Korea in a couple of weeks

You now can rental a SIM for iPhones from SoftBank with unlimited data plan.It worked for me in Tokyo and Kyoto. (Softbank is the only official iPhone provider in Japan)

http://www.softbank-rental.jp/en/phones/