(author here) I didn't have a chance to test how the VPN works, but even if they were using MPTCP the switch wouldn't necessarily be seamless.
Especially how they are using it now - which hands over a Wi-Fi call (which is data) - to a cellular voice call (not data). It's a subtle point, but calls over cellular are not data, so they are not using TCP/UDP/MPTCP/etc.
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(author here) I didn't have a chance to test how the VPN works, but even if they were using MPTCP the switch wouldn't necessarily be seamless.
Especially how they are using it now - which hands over a Wi-Fi call (which is data) - to a cellular voice call (not data). It's a subtle point, but calls over cellular are not data, so they are not using TCP/UDP/MPTCP/etc.
Does this change with VoLTE?
https://en.wikipedia.org/wiki/VoLTE
Yes - with VoLTE, calls are also data.
I see, for some reason I thought it was Voice over IP tunnelled to google via VPN (over wifi or LTE). My mistake.
Exactly. They are doing VoIP over Wi-Fi (VoWiFi), but not VoIP over 3G/LTE (which is VoLTE if you follow certain standards).