There's a lot of back story to this fic, to be sure, and y'know, you're the first person to raise the question as to how Zack got a hold of Tifa. Kudos to you for paying attention.

The short version is that he tried to reach her in Nibleheim, her father gave him her new number, and when Zack realized she was in Midgar, that's when he set about his 'cunning plan'. Zack's a romantic sucker, so of course he wants to help his best friend. :P

This prompt will actually have more parts, and that little bit of information will get addressed, I'm sure. Most likely through Cloud questioning Zack, but I haven't started yet, so I'm not sure how it gets revealed. But, uhm, yeah, rambled enough, eh? :P