In Spanish, to ask someone what his/her number is we say: "¿Cuál es tu número de teléfono?" Here is a table showing the ways in Spanish and English we say "What is your phone number?"
In Spanish, we use "y (number between 1-9)" to say for ex. 77 would be setenta y siete. But you only use that when your writing any number after 30. Video: A real life situation where you would use this is at a party and you are approaching an attractive person to get his/her phone #.