An electronic compass points to magnetic north whether it's in the northern or southern hemisphere.
It is not subject to "magnetic dip" because with an electronic compass there is no physical needle to "drag" on the compass housing. Therefore electronic compasses work equally well in any hemisphere, and even when located very close to the poles.
If it was important to recalibrate when crossing the equator, DJI would say something about it in the manual (and they don't).
Calibrating the compass is not at all related to where you are or distance from anywhere else.
Calibrating the compass enables the Phantom to work out what magnetic influences that the compass reports are part of the Phantom and anything else should be the earth's normal magnetic field.
Unless you are modifying your Phantom, there should be no need to ever calibrate the compass.