I didn't understand. Can you repeat?
Stress 'ca-PI-to'. 'Ripetere' = ri-PE-te-re.
Use this at any point in a phone call when you've missed something or didn't understand. Never pretend to have understood when you haven't.
'Non ho capito' = I didn't understand (perfect past tense). 'Può ripetere?' = can you repeat? (formal). Informal: 'Puoi ripetere?' 'Scusi' before the phrase makes it even more polite. This is one of the most useful phrases for any Italian learner.
Come ha detto?
What did you say?
Simpler, very common.
Scusi, non ho sentito bene.
Sorry, I didn't hear well.
Blaming hearing rather than understanding.
Può fare il lo spelling?
Can you spell it?
Asking for letter-by-letter spelling.
The Italian phonetic alphabet ('alfabeto fonetico italiano') is used on the phone to spell out letters: A = Ancona, B = Bologna, C = Como, D = Domodossola, E = Empoli, F = Firenze, G = Genova, H = Hotel, I = Imola, L = Livorno, M = Milano, N = Napoli... This system is standard in Italian phone communication for names and codes.