Answer:
"¿Cuál es tu número de teléfono?"
Explanation:
Assuming this person is asking a friend, they would want to keep things casual, hence the 'tu'.
One might ask: "¿Cuál es su número de teléfono?" If they were referring to a superior, or someone who should be treated with great respect.
(In this case, su, still means 'your' except in a more formal manner)
If you were to ask someone else what the telephone number of someone else is, you can also say the question "¿Cuál es su número de teléfono?"
(In this case, su, translates to: her or his)
