The big problem with cellphones on planes is that they overwhelm the signalling networks on the ground - when the carrier tries to determine which cell is best suited to handle your call, it basically polls the neighbouring cells for the received signal strength from that particular phone. When you're on a plane, your phone is seen from an awful lot of cells.
Also, some systems (GSM, for instance) relies on your velocity compared to the base station not being too large to allocate you a slot in the time-multiplexed channels. Depending on the geometry of the cell network, you may not be able to connect at all when on a plane - but your phone may create all sorts of grief for the network operator.
Or, put another way - if there really was a non-zero probability that a working cellphone on a plane would cause trouble, the TSA would simply collect the phones during boarding and being caught with one would give you the full terrorist treatment.
Given the abundance of cellphones and the sloppiness of people, I think it is probably safe to say that no commercial flight has taken off without at least one active cell-phone aboard for the past twenty years or so.
And this is in a fiberglass airplane that's basically radio transparent. Now try it in an aluminum airplane at 30,000+ft. I doubt the cells can even hear your phone in the first place.
There's enough radio transparent openings in an aircraft fuselage for it to be a reasonably inefficient faraday cage - say, windows, for instance. (Or are those portholes on an aircraft, too?)
It would be non-trivial to determine the phone's altitude, but you could make a pretty good inference from the number and location of cells reporting that they 'see' it - and the velocity should be a no-brainer to figure out with a reasonable degree of accuracy. Hm.
Just a semi-educated guess. (I haven't looked at cellular systems since I was in university 15-20 years ago.)
Methinks you are probably right.