Ore wa Shiranai Uchi ni Gakkouichi no Bishoujo o Kudoiteita Rashii ~Baito-saki no Soudan Aite ni Ore no Omoibito no Hanashi o Suru to Kanojo wa Naze …

[Kaarme]

I really respect this author, at least so far, for not writing in a triangle. I personally dislike those the vast majority of times, so it's nice it's absent here. When Mai appeared, it kind of looked like she would be interested in Tanaka, but fortunately she's a good wingwoman instead.

I'd recognise a friend's voice in a crowd no problem. It's just some plot magic anyway

Yes. And this is manga, a silent medium. It has got no voice. That's an absolute requirement for this to work. In RL, this would assume Saitou speaks either above or below her normal vocal frequency range as Hiiragi, to fool Tanaka. Just how tiring would that be, assuming their shifts are at least a few hours and seeing how it's a customer service job with lots of talking? Besides, why would have she have done that before Tanaka joined the staff? If she only started that after Tanaka joined, wouldn't the other staff members and regular customers be baffled? So, yeah, it wouldn't happen. So, this only works in a silent medium.