It depends on where you are relative to the towers.
In the UK there is rarely a signal above 2k-3k.
In southern France, near the mountains, I was able to get a dial-up data call (for internet weather) at 8k. The call lasted maybe a few minutes.
I assume they had towers high up. In 2002 I saw people using mobile phones on top of Mt Kinabalu in Borneo after walking up there; 16000ft.
Never tried voice calls...