Assuming you're asking about a phone number, then you answer thus: mi numero de telefono es XXX-XXXX. However, literally my answer would probably be: my telephone is the black one.
Cual es tu telefono = What is your telephone number
cual es tu numero de telefono = what is your phone number cual es el numero de telefono = what is the phone number
Cual es tu numero de telefono in English literally translates to "What is your number of phone?" Which really means "What's your phone number?"
What is your cell pnone number
It means "what is your telephone number?" in Spanish.
It means: What is your phone number?
Cual es tu nacionalidad means "what is your nationality". So you would reply with your nationality ("Mi nacionalidad es....)
cual es tu means 'what is your....' (informal)cual es su means 'what is your....' (formal)
cual es tu apellido means what is your surname so you would with respond with your last name.
Which is your sport.
Mi nombre es...
Cual es el poblacion de tu pais means What is the population of your country