Hey, been a long-time lurker here, and I can't remember a one-shot for the life of me.
I believe that Girl A gives her phone number to Girl B saying that it's a guy's phone number that's interested in her, and Girl A "acts as a go-between" for them (even though the guy doesn't even exist). Later on Girl B tells Girl A she hates liars, which is what Girl A is. Girl A confesses via text (so, as the guy) and gets no response. Then, when they're together, Girl A blurts out that she's the "guy" and runs away. Girl B catches up and asks if the confession via text was a lie, Girl A shakes her head, and Girl B pulls out her phone and texts the reply of "I love you too!" to Girl A.
Thanks for the help!