The mana value of a spell cast without paying its mana cost is determined by its mana cost, even though that cost wasn't paid.

If you cast a card "without paying its mana cost," such as with suspend, you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those if you want to cast the card.

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Suspend is a keyword that represents three abilities. The first is a static ability that allows you to exile the card from your hand with the specified number of time counters (the number before the dash) on it by paying its suspend cost (listed after the dash). The second is a triggered ability that removes a time counter from the suspended card at the beginning of each of your upkeeps. The third is a triggered ability that gives you the option to cast the card when the last time counter is removed.

You can exile a card in your hand using suspend any time you could cast that card. Consider its card type, any effects that modify when you could cast it (such as flash) and any other effects that stop you from casting it (such as from Meddling Mage's ability) to determine if and when you can do this. Whether you could actually complete all steps in casting the card is irrelevant. For example, you can exile a card with suspend that has no mana cost or that requires a target even if no legal targets are available at that time.

Cards exiled with suspend are exiled face up.

Exiling a card with suspend isn't casting that card. This action doesn't use the stack and can't be responded to.

If the spell requires any targets, those targets are chosen when the spell is finally cast, not when it's exiled.

If an effect refers to a "suspended card," that means a card that (1) has suspend, (2) is in exile, and (3) has one or more time counters on it.

If the first triggered ability of suspend (the one that removes time counters) is countered, no time counter is removed. The ability will trigger again at the beginning of the card's owner's next upkeep.

When the last time counter is removed, the second triggered ability of suspend (the one that lets you cast the card) triggers. It doesn't matter why the last time counter was removed or what effect removed it.

If the second triggered ability is countered, the card can't be cast. It remains exiled with no time counters on it, and it's no longer suspended.

Due to a recent rules change to suspend, you are no longer required to cast the suspended card as the second triggered ability of suspend resolves. Instead, as the second triggered ability resolves, you may cast the card. Timing permissions based on the card's type are ignored. If you don't cast the card, it remains exiled with no time counters on it, and it's no longer suspended.

If the creature's power is negative, you still add its power and toughness to determine how much life is lost. If that total is negative, its controller doesn't gain or lose any life. For example, Phthisis destroying a -1/3 creature would cause a loss of 2 life. Pthisis destroying a -4/3 creature would cause no loss of life.

If the target creature is an illegal target by the time Phthisis tries to resolve, the spell doesn't resolve. No player loses life. If the target is legal but not destroyed (most likely because it has indestructible), its controller does lose life.