I'm surprised that (it seems) google isn't using Multipath TCP to carry the VPN traffic to google. This would allow it to switch seamlessly between LTE and Wifi and in theory even LTE and LTE while maintaining the VPN connection and thus the call.
In fact, Apple uses this tech for Siri to reduce latency on voice queries.
In fact, Apple uses this tech for Siri to reduce latency on voice queries.