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Hangouts is smart enough to switch between the wifi and data on the fly even, though sometimes the call wont survive the transition. I would recommend may be getting one of the LTE hotspots on the sprint network and putting it in your car if you do a lot of calls or streaming music. You can get a gig with that.
Also note the global sim is a UK sim so your data gets VPN back to UK and then has to come back to the US. This can mess with hangouts by adding a small delay on the call, which is also why i recommend grabbing an LTE hotspot for your car.
Also being a UK sim you will have to make sure all hangouts calls are dial with full country code like so +12345678901. Shameless plug, my app will handle automatically adding country and area for you when using your native dialer and redirect the call to hangouts.