The calendar underneath is the live one, so take whatever slot suits you and it is held the moment you pick it. Bring last year's return if you can lay hands on it, because the conversation turns useful the instant real figures are on the screen instead of general principles, and twenty minutes is long enough to walk the levers that were open on that return and the ones still open in the year you are in now. Nothing is prepared in advance and nothing is sold to you on the call. You hear what the numbers say, in plain words, and then the next move is yours. If the calendar refuses to load on a phone, the link under it opens the same booking page in a new tab, and the number works just as well if you would rather speak to somebody today.