https://fi.google.com/about/
Right now will only be available for the Nexus 6 phone. Maybe just for U.S. customers, but hard to tell. About $30/mo covers use in 120 countries. Haven't read all the details. Look for independant news postings on the net as well as going to Google site, above.
Make use of T-M's UMA/wifi free calling from any place in the world with access to wifi. I use an LG G6, wife an S7)
A/o Oct 20, 2013 no need for intl prepaid as T-Mobile U.S. includes voice roaming at 20ยข/min (in and out)., unlimited text (in and out), and unlimited data in 140+ countries.
My Plan -[6 lines] U.S. T-Mobile unlimited minutes (incoming and outgoing), unlimited text, fast data on each line. that $145/mo. total! . (In U.S. no surcharge for calling a cell.) If a line exceeds 2G of data in a month, pay $10 more for that line. [That only happens a couple times/year.