You've got the wrong number.
Stress 'sba-GLIA-to'. The 'gl' makes a soft palatal sound.
Say this when someone calls you by mistake — they've dialled the wrong number. It's polite to let them know rather than simply hanging up.
'Ha sbagliato' = you have dialled incorrectly (formal past tense). 'Numero' = number. The informal version: 'Hai sbagliato numero'. You can add: 'questo non è il numero che cerca' (this is not the number you're looking for).
Penso che abbia sbagliato numero.
I think you have the wrong number.
Softer, more tentative version.
Qui non c'è nessun Bianchi.
There's no Bianchi here.
Explaining who isn't here.
Quale numero cercava?
What number were you looking for?
Trying to help the caller.
Italians typically respond to wrong numbers with polite patience rather than frustration. The common response 'figursi' (don't mention it) or 'non si preoccupi' (don't worry) reflects the Italian social value of gracious interaction. Slamming down the phone on a wrong number would be considered very rude ('sgarbato').