All i know is that it is a stack effect that can increase your pet dmg and you can use necrosis , plaguebolt , aoe ability in necro tree to make more stacks on target + debuff should buff pet dmg but there is no detailed information
Seems like this would not have much usefulness in open pvp . i can see it in raids if you can stack with other mages deathly calling it could really add up. Doesn't seem to add much damage in solo PVE .Things happen so fast in open PVp it would be useless.
Unless it was changed in 1.2, stacks of Deathly Calling also count as cleansable debuffs. While this may seem like a bad thing, it actually means that your more important debuffs, like Necrosis and Looming Demise, are less likely to be cleansed.
Actually Deathly Callings add quite a bit to PvE DPS of the Zealot and caster. The problem is it isn't enough and you can do very well with a partial Necro spec and just maintain Necrosis and Looming Demise while using the Zealot since two of his attacks don't need Deathly Calling to hit hard.
Looming Demise needs two changes to make a full Necro spec with Plague Bolt worth it. First the pets need to consume Deathly Callings faster. Second Deathly Callings need to boost pet DPS more. Do those and you'll see 51 point Necromancers in raids.