I think the Question is "What is the minimum number to find out for certain what someones phone number is (a nine digit phone number)?"

Is your Phone number less than 500 000 000?
If Yes then
Is your phone number greater than 250 000 000?
If Yes then
Is your phone number greater than 375 000 000?
etc

The minimium number to ask to find out what it is is then 30. (I think its supposed to be 32 but my calculator says 30)