I am not sure whether Project Fi uses this technology but Apple have been using ...

taspeotis on June 29, 2015 | parent | context | favorite | on: Project Fi Review: Cell Service from Google

I am not sure whether Project Fi uses this technology but Apple have been using Multipath TCP [1] for a while. I think Apple's implementation is limited to data, in the conventional sense, rather than data+voice.

[1] https://en.wikipedia.org/wiki/Multipath_TCP ("Apple iOS 7, released on September 18, 2013 is the first large scale commercial deployment of Multipath TCP.")

LeoPanthera on June 29, 2015 [–]

If you have an iPhone on T-Mobile (US), calls will automatically switch between cellular and "wi-fi calling".