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

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 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.")