Reminded by the Vickers Funbus thread.
In the US we were forbidden from telling the US controllers if we were flying above FL 450; strangely no such limitation applied in UK.
As we entered US airspace from Canada we were instructed to descend and maintain FL 310. Now we never really flew that low and had no idea of the fuel burn down there. (In fact it was probably better) So the skipper, worried we would be low on fuel at destination requested higher.
"Roger Sir, you are cleared FL 510."
Mmmm, what now? Only one answer really.
"Roger, climbing" I think we still declined to confirm we were at a level above 450
Another occasion, I am told, Socttish was warning aircraft that Benbecula was active to 50,000ft. Rather bored Shack crew acknowledge and revised their ETA Kinloss by a couple of hours. Next Scottish warned a homebound Vulcan en route from Goose. "Roger, climbing."