I've Got Your Number by Sophie Kinsella

Londoner Poppy Wyatt is frantic after she's lost her engagement ring while having tea with friends at a hotel. The ring is a family heirloom of Poppy's fiance, Magnus and she doesn't know how she's going to tell him about what happened. Things go from bad to worse when her mobile phone is stolen while she's outside the hotel. Seeing a cell phone in the garbage, she decides to grab it and use it as her own. Poppy figures she can't be without a phone right now because she's given out her information to everyone, in case they come across the ring. The phone actually belongs to the assistant of businessman Sam Roxton, who's quit her job. Poppy convinces Sam to let her keep the phone for a while, as long as she forwards all his e-mails. But keeping Sam's phone isn't as easy as Poppy thought. Slowly, Poppy becomes involved in Sam's personal and professional lives--and he in hers. And when both have personal crises, they realize it's only the other that can really help them. Another enjoyable novel by Kinsella.