I excluded Brooke and Franky because I'm not familiar with them.
Rornora Zoro, to others, was a strange man for he was indifferent to the extent of being oblivious, and laid back to the extent of being unconcerned. Few knew that Roronora Zoro had his own reasoning, his own rules he lived to abide by and thanks to said reasoning and rules, the world that was a mix of complicated choices, options and results, was actually quite simple for the swordsman. Such reasonings and rules were observed even during simple tasks such as when dealing with his crew mates.
When Robin called, it was business-like for there always was an objective or reason that concerned the welfare of the crew. Thus Zoro would answer after a few moments of pause while he stirred himself out of sleep.
When Chopper called(or rather, shrieked as he dashed towards the swordsman), it was usually because Zoro was in the middle of his four hundred and fifty seventh push up, an action that Chopper had deemed inappropriate for a man who just suffered internal bleeding in parts of the body that shouldn't be bleeding, and broken bones in place people normally lived on without knowing they had. Thus Zoro would roll his eyes before he explained to the reindeer that he was fine and the two barrels of beer cured everything, before giving in to the glistening tears from the reindeer who took that comment as a personal insult.
When Sanji called, which he didn't save before meal times, it meant something very very bad had happened to Zoro for the cook to be shouting Zoro's name instead of one of the many nicknames Sanji so kindly made himself. And in those times Zoro usually wasn't in the state to answer. If he was, the answer was usually something completely idiotic and stupidly heroic but so maddeningly Zoro that no one would be able to push the subject further(ex. Nothing happened).
When Usopp called-excluding dire situations that included, but were not limited to, tornados, hurricanes, sea monsters, marine war ships, and Luffy's sugar rushes-it was usually to provoke Zoro to play tag, hide and seek or whatever new activity Usopp thought up of. Thus Zoro snored on.
When Nami called, it wasn't good. Whatever it was, it was never good. And so Zoro usually slept on, snoring a little louder before breaking off abruptly and waking up in a foul mood, muttering blasphemy about 'the damn witch who should drop to the deeper halls of hell for threatening to double a debt that shouldn't even exist in the first place'.
When Luffy called, Zoro answered. Whether it was a war cry, shout of surprise, or a strangled whimper so low and so soft even for Chopper's ears, Zoro answered.
It was as simple as that.