The rulebook isn't very specific about stuff like this. The spellcasting rules (p16) don't explicitly say it's impossible to pay more Sanity than you actually have. I used to think that this wouldn't matter, because as soon as you paid 13 Sanity with a 3 Sanity character, you'd go insane and this would prevent the spell 'resolving'. (You can tell I used to play MtG, can't you?).
But actually, Kevin Wilson's official answer on the old forums was that you could still make the Lore check and cast the spell, even if the sanity cost had just sent you to the Asylum. Of course, in the cases of most spells, this wouldn't help, since you'd have been removed from the situation in which the spell would be useful anyway.
Personally, I think the implication of many of the game's rules is that a 'cost' is distinct from a 'loss' and that if you want to pay a cost or spend something, you have to actually have that something. Even if you don't think there's any evidence for this in the scripture, it's surely an essential house rule.
Call Ancient One is an odd spell though. The order in which you perform the various actions necessary to cast the spell seems to imply that when you discard the trophies, that sets the value of X. The sanity cost becomes a fixed number, and you can apply modifiers to that number just like any other spell's Sanity cost. If you discard 13 trophies, you need someone who can pay 13 Sanity. Going with the interpretation of the rulebook above (which is the only sensible one I reckon), I don't think that's possible to pay 13 San with one character.
But you can get close. Harvey Walters (Max San 7), with:
Duke the dog (+1)
Open The Third Eye (+1)
The right Blessing of Nodens (+1)
The 'Changed' card from Kingsport (+1).
That gets you to 11, but it's pretty difficult to get all those things, particularly the specific Nodens card! I had a feeling that there's also an encounter somewhere which adds three extra temporary max sanity, but I can't find it now. Anyway, 11 Sanity paid will kill Yig, Ithaqua and Nyarlathotep. If you can get three more somehow, it'll defeat anyone. You'll need Clues to pass the Lore check of course, or the skill that adds 1 success.
What you really need instead.... is a Crystal of the Elder Things. With one of those, anyone can cast call AO for 14. Assuming you've got 14 trophies of course. You know, now that I think about it, it's actually a pretty good way of winning, if you're willing to trawl through the item decks.