MB, depending on the type of phone, there is a lot of (admittedly circumstantial) evidence that they do interfere with some aircraft equipment. Studies are continuing.
There is, however, another problem, which is that at ground level, your phone is unlikely to "see" more than two repeater stations at once. At altitude, however, it can "see" lots, and every one competes to provide you a service. Not a problem from your point of view, but is means that you are severely degrading the service other network users receive, since each repeater station can only serve a limited number of phones at once.
Both the above are the reasons why the Telecommunications Agency has decided to make it illegal to have a phone switched on in the air. Therefore, I appreciate that you may not have experienced any problems, but you are breaking the law.