Looking at the antenna lobe patterns for cell phone towers, with the primary lobe being horizontal, it really surprises me that any connection was made. I have tried in the past to use a cell/mobile phone from low altitude in a lightplane, with very poor results.
I've had a similar result with the Australian GSM phone system in my recreational flying - not that I've used it much. (Rec flying is for fun not for phones). However once when I really did need it at about 8,000ft, it was useless. Full scale signal strengths on the meter, but the network unavailable tone was all I could get. When I had descended to about 4,000 ft, and thought to check again, it worked perfectly. I wondered whether at the greater altitude, signals from local cells (below) were being swamped by those on the near horizon, and that these may have been too far away to meet the timing loop requirements of GSM, something which I had encountered before in the GSM regime - both in the air and on the ground.
Nowdays I'm in the 3G system, but I haven't had the chance or the need to see what happens there in the air. I suppose I should sometime, just in case it's important one day.