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Some spell effects, like flesh to stone, have no duration, its an instantaneous effect (see bellow), which are not subject to antimagic zone because the spell already happened and what remains is the result of the spell.

Another example is the damage caused by a Fireball. The spell itself wouldnt work inside an antimagic zone, but the damage caused is instantaneous, once the spell is cast and the damage is taken, the remaining effect (health lost, burn wounds, ego hurt, etc) is not magical per se.

Some spell effects, like flesh to stone, have no duration, are instantaneous effects (see bellow), which are not subject to antimagic zone because the spell already happened and what remains is the result of the spell.

Another example is the damage caused by a Fireball. The spell itself wouldnt work inside an antimagic zone, but the damage caused is instantaneous, once the spell is cast and the damage is taken, the remaining effects (health lost, burn wounds, ego hurt, etc) are not magical per se.

Shapechange mimics the spells from polymorph school, which do have a duration.

Shapechanging in general mimic the spells from the polymorph school, which do have a duration.

Some spell effects, like flesh to stone, have no duration, its an instantaneous effect (see bellow), which are not subject to antimagic zone because the spell already happened and what remains is the result of the spell. The "stone" is not magical, its real.

Shapechange mimics the spells from polymorph school, which do have a duration.

Called creatures won't get dispelled either, because the spell that bought them to the material plane already did it's work, the creature is physically here and won't disapear in an antimagic zone. But a summoned creature will disapear as long as it remains "inside" the zone.

Calling: a calling spell transports a creature from another plane to the plane you are on. The spell grants the creature the one-time ability to return to its plane of origin, although the spell may limit the circumstances under which this is possible. Creatures who are called actually die when they are killed; they do not disappear and reform, as do those brought by a summoning spell (see below). The duration of a calling spell is instantaneous, which means that the called creature can't be dispelled.

Change Shape (From universal monster rules) says: This ability functions as a Polymorph spell, which has Duration 1 min/level (D).

But Change Shape specifically clears it up that the creature can remain in that form indefinitely unless said otherwise. Meaning that it has a long-lasting duration.

Instantaneous The spell energy comes and goes the instant the spell is cast, though the consequences might be long-lasting.

Because as long as the effect has a duration and is magical, it will stop working inside an antimagic zone. And once you walk out of it (or the zone walks away) the magical effects return.

Some spell effects, like flesh to stone, have no duration, its an instantaneous effect (see bellow), which are not subject to antimagic zone because the spell already happened and what remains is the result of the spell.

The "stone" is not magical, its real.

Another example is the damage caused by a Fireball. The spell itself wouldnt work inside an antimagic zone, but the damage caused is instantaneous, once the spell is cast and the damage is taken, the remaining effect (health lost, burn wounds, ego hurt, etc) is not magical per se.

Shapechange mimics the spells from polymorph school, which do have a duration.

Change Shape (From universal monster rules) says: This ability functions as a Polymorph spell, which has Duration 1 min/level (D).

But Change Shape specifically clears it up that the creature can remain in that form indefinitely unless said otherwise. Meaning that it has a long-lasting duration.

Instantaneous The spell energy comes and goes the instant the spell is cast, though the consequences might be long-lasting.

Called creatures won't get dispelled either, because the spell that bought them to the material plane already did it's work, the creature is physically here and won't disapear in an antimagic zone. But a summoned creature will disapear as long as it remains "inside" the zone.

Calling: a calling spell transports a creature from another plane to the plane you are on. The spell grants the creature the one-time ability to return to its plane of origin, although the spell may limit the circumstances under which this is possible. Creatures who are called actually die when they are killed; they do not disappear and reform, as do those brought by a summoning spell (see below). The duration of a calling spell is instantaneous, which means that the called creature can't be dispelled.