Mobile phone frequencies are not quite line of site only, they can also bend or creep to a small extent around objects, filling a lot of void spaces. This along with
1) Dual frequency operation and
2) the straight relections off metal (and hidden metal such as concrete)
means that the main limitation is range owing to the power of the phone and the repeater.
So cross channel reception stops about 5 to 10 miles offshore normally
Its still not regarded as safe to use a mobile phone in either a car unless it has an external aerial, (or I suppose a small steel ship), as the microwaves do get concentrated near the centre of the box by virtue of focusing, so your bodily fluids will still get heated!
The debate on long term damage is still ongoing so use them wisely (foil underpants are available in Japan for protection)
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