How to check which cellular network bands are supported by iPhone
EU phones will all be GSM.
You cannot cross the divide between GSM &amp; CDMA. 

Read this for a long but simple explanation - http://uk.pcmag.com/cell-phone-service-providers-products/11593/news/cdma-vs-gsm-whats-the-difference
… though having read that twice &amp; my head is still spinning - I'm still not actually sure if the fact that 3G = WCDMA means it will work….
I always used to have to have a different phone to use in Japan, but that was several technology generations ago.

https://xkcd.com/927/  ;-)