Just bought a Fiat Punto and read the book. It says that mobile phones without a proper external antenna can interfere with electronically controlled items, which could result in danger to the occupants. (There was no list but this may include engine control and possibly the electrical power steering as now fitted.)
Taking it that this is true and not just to forestall eager lawyers, aircraft have quite a few more systems, which it would be as well not to have interfered with.
Fiat may have suggested a good solution - a proper external antenna fitted to passenger aircraft, with a shielded connection for mobile phones.