Last Updated: August 14, 2020
Project Fi is a new kind of phone service....
This service can be used with a broad range of Android phones as well as iOS 11 (and later) iPhones. It still works best on phones designed for the service, such as the Google Pixel series.
This mobile virtual network operator (MVNO) service preferentially uses wi-fi connections for all services, and if wi-fi is not available, can automatically select and switch between T-Mobile and Sprint mobile LTE services. Data is $10 per Gigabyte; and you pay only for what you use.
International 3-G and LTE data (140 countries) is also just $10/GB. Free text messaging is included. Voice calling over wireless, 3G and LTE networks is relatively inexpensive, $0.20 or less to/from most of the supported countries.
I started with the service on July 30, 2015. So far, I've used the service in the US, Australia, Canada, Croatia, England, Greece, Ireland, Italy, Montenegro, New Zealand, Northern Ireland, Portugal, Scotland, Spain, Tonga and Turkey. It works as advertised, with a few minor glitches in international use. When entering a new country or "service area" and attempting a voice call, the phone would often state that it wasn't provisioned. Turning the phone off and back on cures that. Total international "roaming" cost for a two week period was less than $2 even with frequent use of email, navigation, news browsing and social media. On recent trips, LTE data speeds were better than hotel wireless speeds in many countries.
Some countries/places still aren't supported. French Polynesia and the Cook Islands, for example.
One nice feature is automatic voice-to-text "transcription" of voice messages. Does a great job, very few mistaken words, and every call-back number left in a voice mail so far has been correct.
I highly recommend the Google Project Fi service for general use and international travel.