I'm in an area of complex controlled airspace. One agency, a big city tower, controls the airspace from the surface up to 4000'. Above that, the terminal controller is responsible for airspace up to to 12,500', and Centre controls everything above 12,500'. I've been working in high level airspace, but have now descended and am talking to terminal. They know my intentions: to land at an airport just past the big city. They've told me to maintain 6500' VFR. I do this, following the route they specify, following landmarks, then an assigned heading and then direct a nav aid. They have traffic below me, but I must be past it now, because they tell me I'm cleared to 4500', contact tower for lower.
I'm expecting this, so I have the tower frequency already tuned on standby. After I acknowledge the instruction, I push the button to switch to tower frequency. I don't call immediately, because I need to listen for a moment to make sure I'm not interrupting a conversation. I start descent and then call tower, giving them my call sign and that I'm passing "six thousand two hundred for four thousand five hundred." The first number is the altitude they can crosscheck with my transponder readout, and the second number is the altitude I was cleared to.
The tower controller says that he is not responsible for that airspace, and that I should call terminal. "Terminal sent me to you," I say. It's not uncommon for there to be miscommunication between tower and terminal. The tower controller tells me to go back to terminal then, because 4500' is not in his airspace. And now I know the problem: although my destination should be encoded on my radar blip, either the tower controller hasn't looked at it or it's been miscoded. I tell him the airport I want to land at and he clears me down through his airspace for it, grumping that I should have asked for that in the first place.
I wonder out loud to my coworker about who put a frog in the tower controller's cocoa, and continue my descent. But I see the ambiguity there. Pilots say, "Niner thousand for six thousand five hundred," both to request a descent to six thousand five hundred and to indicate a descent in progress, with the intention of stopping there. The latter could be a clearance limit in uncontrolled airspace, or just a pilot stating her intentions, if the descent will take her out of controlled airspace. I could have said "six thousand two hundred requesting descent for
UK-trained pilots will jump in now with radiotelephony rules, because their language and training is more precise. I agree ours should be, and am somewhat surprised that in thousands of hours of flying this is the first time this particular ambiguity has bitten me. I suppose there have been numerous times when the second controller has taken my statement of an existing clearance to be a request, and recleared me to that altitude, but they didn't stand out enough for me to realize the ambiguity.
Controllers are damned smart. I think most of the time they just figure it out.