It’s 1956, and busy New Yorkers use "Susanswerphone," a telephone answering service, to ensure they’ll never miss a phone call again (“Bells Are Ringing”). One operator at Susanswerphone, Sue’s cousin Ella Peterson, particularly enjoys listening in on people’s lives. Ella adds some interest to her own humdrum existence by adopting different identities – and voices – to serve her clients. Those clients include Blake Barton, an out-of-work Method actor, Dr. Kitchell, a dentist with musical yearnings but no talent, and struggling playwright Jeff Moss, who desperately needs a muse. Though she’s never laid eyes on him, Ella realizes she’s fallen in love with Jeff ("It's a Perfect Relationship").
Jeff struggles to finish writing The Midas Touch, the first play he's written since his collaborator departed ("Independent"). The night before an important 9 am meeting with his producer, Jeff calls Susanswerphone to request a wake-up call. He asks Ella – whom he regards only as “Mom,” a kindly old lady – for some creative inspiration (“You’ve Got To Do It”). Meanwhile, Sandor, Sue's rich boyfriend, reveals plans to a group of gangsters to use Susanswerphone as a bookmaking operation, by pretending to be a record seller and taking orders for "symphonies" as code ("It's a Simple Little System").
Ella sets off towards Jeff's apartment to help him write the play, but she is intercepted by a policeman who suspects that Susanswerphone is a front for an "escort service." Ella tells him she’s just helping someone in need ("Is it a Crime?") and he lets her go. When Ella arrives at Jeff's place at 7am, she discovers him asleep on his couch. He suddenly awakens, so Ella pretends claims to be his neighbor, Melisande, who mistakenly entered the wrong apartment. She helps motivate him to write his play and a romantic spark is lit (“Better Than A Dream”).
Jeff’s play is well received, so he and Melisande/Ella celebrate by greeting strangers on the street (“Hello, Hello There”). Jeff is smitten ("I Met a Girl"). Detective Francis, who continues to investigate Susanswerphone for fronting an escort service, follows Ella to Jeff’s apartment one night. In the apartment, Jeff and Melisande/Ella declare their love ("Long Before I Knew You"), and just as they embrace to kiss, Francis snaps a photograph.
As Ella anxiously anticipates an evening date with Jeff, her friend Carl calms her nerves with a sizzling dance ("Mu-Cha-Cha"). Ella meets Jeff in Central Park, where he serenades her in front of admiring crowds (“Just In Time”). Jeff takes Melisande/Ella to a fancy show biz party, where he proudly introduces her to the pretentious, rich guests ("Drop That Name"). Ella, feeling out of place, realizes that Jeff is in love with an illusion. She leaves without saying goodbye ("The Party's Over").
Sandor, in need of some cash to cover his racing debts, convinces Sue to lend him $6,000, and the two plan to run off together (“Salzburg”). Jeff, despondent over Melisande’s sudden departure, seeks her out at their favorite nightclub (“The Midas Touch”). At the club, Jeff, struggling actor Barton, and dentist-turned-musician Dr. Kitchell realize they’ve all been inspired by the same woman, but they don’t know how to reach her. They all exchange phone numbers, and Jeff leaves to find his Melisande. Police officers enter and immediately take Barton and Kitchell for questioning.
In Bay Ridge, Jeff has no luck finding Melisande. He calls Barton and Kitchell, reaching Susanswerphone both times. He finally puts it all together, realizing that Melisande is “Mom.”
At Susanswerphone, Ella decides that her life is too complicated, and she packs her backs to flee to her previous employer (“I’m Going Back”). Sandor, pursued by two henchmen, enters, wondering who changed his order for “Beethoven’s Tenth” to “Beethoven’s Ninth.” Ella admits she made the correction, since Beethoven only wrote nine symphonies. Francis and the police enter and arrest Sandor and the henchmen for operating a booking ring. Jeff arrives and declares his love for Ella/Melisande/Mom. As Ella’s other clients arrive, she introduces them – by phone number – and they all join in celebration (“Finale”).