Yeah, I really don't know for sure, I just know that my Fi phone behaves oddly when crossing into Mexico and Canada compared to my Verizon phone a bit ago and I

Yeah, I really don't know for sure, I just know that my Fi phone behaves oddly when crossing into Mexico and Canada compared to my Verizon phone a bit ago and I'm guessing it has to do with some of the less typical cost optimization Google does wrt carriers. I don't know a lot of technical detail about how Fi does that and I don't think there's much info about it out there in general, but my theory at least makes sense to me... well-standardized VoLTE and eUICC are pretty recent developments (the same capabilities have existed for much longer but in the form of various proprietary solutions). So this could explain why, early on, Fi was limited to only a few Android devices with bespoke kernels, while nowadays (as VoLTE and eUICC have become much better supported) it works correctly on a much broader range of devices. VoLTE, for example, has been around in concept for longer than LTE, but it has had a complex standards history that I did not try to really explain because it would get lengthy. "VoLTE" and "IMS" can actually refer to different systems (not all VoLTE uses IMS) but it all gets sort of vague and contextual. LTE phones with VoLTE capability have been around basically as long as LTE has been in service in the US, but for a long time were mostly proprietary systems that required per-carrier support. This is still true to some extent but US carriers and device vendors are now much more converged on an IMS-esque architecture than they were even just a few years ago, so it's now a lot more likely that you can e.g. switch to a different carrier and still have VoLTE work... although not guaranteed! One hint that might help reveal how Google Fi handles this is the source of web traffic on my phone... web requests from my phone in Mexico are coming from an IPv4 address registered via ARIN to Airlinq with a contact address in the UK! Airlinq has some kind of relationship to Hong Kong ISP Hutchison and it seems like the takeaway is that they offer a "global cellular mobility" solution. My guess is that Telcel is being used as the mobile provider (I did some checking of intercept recordings which confirms my voice calls at least are going via Telcel) and IP traffic is being routed via Airlinq, whom Google presumably contracts to handle international IP connectivity. Normally, in the US, my traffic emerges via IPv6 from T-Mobile. This is an indication that my traffic is not being tunneled back to the "home" provider as is the typical case with roaming.