Here in OZ 2G and 3G are now discontinued as they have not been reliable networks "though 3G is still active it won't be long before it is disconnected for good.
Your services in Japan maybe excellent "except your hills 'mountains' maybe a problem"we here are running with 3G and 4G but Australia is a big place and I live in the Western side and phone reception is hard when you leave the big city.
So to answer your question, we here do not rely on phone networks for GPS navigation, most use Nav units when going outside the City limits.
Garmins/Tom Tom or any other unit that uses the GPS nat Satellites.
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