When you activate the effect of “Call of the Mummy”, you can Special Summon a high-level monster from your hand like “Giant Axe Mummy” or “Vampire Lord”, but not a Special Summon-only monster like “Fushioh Richie”.
You can only activate “Call of the Mummy”’s effect during your Main Phase.
You select a monster from your hand when you resolve “Call of the Mummy”’s effect, not when you activate the effect.
If you have multiple copies of “Call of the Mummy” in play, you can only Special Summon 1 Zombie because when you try to activate the second “Call of the Mummy”, you now have a monster on your side of the field so its effect does not apply.
Netrep Rulings:
If “Call of the Mummy’s” effect is chained with a card that will destroy “Call of the Mummy,” “Call of the Mummy’s” effect will disappear.