I've Got Your Number: A Novel by Sophie Kinsella

I've Got Your Number is the story of Poppy, who, misplaces her antique engagement ring seconds before her phone is taken. Incredibly, she finds a phone that someone has cast off in the trash. In exchange for keeping the phone, so she can be contacted should the ring, she agrees to forward the abundant texts and emails. She becomes a kind of unpaid personal assistant to Sam, the owner of the ditched phone. In the end Sam and Poppy become ensnared in one another's lives.
5 Stars