<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by 777Brit:
Maybe, but he/she is also quite correct.
Digital (GSM/PCN) networks will work up to about FL250, Analogue networks are good up to about FL350.
Digital networks don't want handsets to be used because they would be switching cells every few seconds or minutes. The networks are not designed to have handsets literally flying across them at over 500mph. This constant cell-switching can 'crash' the digital cell and reduce network coverage.
Of course, remember that there's no proven evidence of cellular devices causing uncontrolled flight control surface deviations or interference with actual navigational aids. The most intereference comes from the high-pitched noise heard in the pilots headsets whenever a cellular telephone is used. </font>
It may be true that you can get the phone to work at altitude, but you create a lot of grief for the cellular network - in particular other users of the network. In analog systems and most digital systems (with the possible exception of CDMA), the systems are designed to reuse the same frequencies every so many cells. When you use it at altitude, your assigned frequency can blanket a very large area, being strong not only in the cell that you're using, but in many other cells where the system has assigned another user to your frequency. Since the cellular system has no way of knowing that you're at altitude, it's quite likely that your frequency will be assigned to other users (perhaps many) whose calls won't work properly.
This is why, at least in the US, it's against FCC (Federal Communications Commission) regulations to use a cellphone in an aircraft while airborne. So, even if the FAA were to determine that there was no danger to the aircraft, it would still be illegal in the US to use one while in flight because of interference to other users of the cellular system.