We all run into questions we don’t know the answer to or aren’t prepared for sometimes.  The key here is to not mislead voters – even by accident.  Just acknowledge that you are only a volunteer and direct them to get more information on the campaign's website.
 
If you are making calls from a campaign office and you feel like you’re stuck on a question a voter asks, there is nothing wrong with asking a member of the campaign staff how they want you to answer.