"Do you know my phone number?"
Translation:¿Tú sabes mi número de teléfono?
In general, saber is used for knowing facts, information, how to do something.
Conocer is for meeting and knowing people (that's when it uses the "a"), being acquainted with them, and for knowing (being familiar) with places.
More at studyspanish.com (Grammar Unit Five).
I forgot to put the 'de' in "número de teléfono" and it said the same thing, but when I inserted the 'de', it worked with 'teléfono'. Duo doesn't always get it's corrections right, but it tends to be relatively good with accepting or rejecting.