I've Got Your Number by Sophie Kinsella
Uh Oh, Poppy has lost her engagement ring from Magnus that's been in his family for three generations, plus her phone's gone walkabout too. When she spots an abandoned mobile in a bin it seems it was meant to be. Finders Keepers! Except the phone's owner, elusive businessman Sam Roxton, doesn't agree. Woops. Buy this book now.