Your birth year should be more than satisfactory for their demographics (which really has NOTHING to do with the stated intended purposed of the cencus). And what practical use/need do they have for your phone number?

Really, they only need to identify 3 things:
Number of individuals in the home
Number of individuals per age group (0-5/6-12/13-18/19-25/26-34/35-45...
Estimated annual income of the household

Anything beyond that is just being nosey and intrusive.