Summon Undead Problem Solved?

[Thanee]

Does summon undead actually call the skeleton from somewhere? I thought it just created it.

It summons it, it does not call it. These two are very specific terms within the D&D rules and should not be confused.

Anyways, the spell does not create the creature, but summons it from... somewhere (probably the negative energy plane), just like the Summon Monster spells. It is generally assumed, that every possible creature exists somewhere for this purpose. Who knows, maybe there is even a demiplane with a bunch of busy necromancers who happen to have nothing better to do, than create undead for this purpose.

Bye
Thanee