What is your phone number?
'Telefono' — 'te-LE-fo-no'. Four syllables; stress on the second.
Ask when you want the individual driver's number for a future direct booking, or when exiting a taxi to have a way to contact them about forgotten items.
'Qual è' means 'what is'. 'Il suo numero' uses the formal possessive 'suo' (your, formal). This is a polite and natural way to ask for contact information from a service provider.
Posso averla di nuovo domani?
Can I have you again tomorrow?
Informal way to rebook the same driver.
Manda il numero al mio telefono?
Can you send the number to my phone?
Asks for the number via text.
Qual è il numero della centrale?
What is the dispatch centre number?
For booking through the company rather than the individual.
Many Italian taxi drivers hand out personal business cards ('biglietti da visita') to regular clients. This direct-booking relationship is common in Italy and is completely legal — the driver still uses the official meter.