Phone number so you can call them, and also their email address. Best to have at least 2 points of contact in case one doesn't work.

Be aware that as your site becomes easily visible online you'll get spammers too. So adding a field where there is a definite answer that needs to be given, for example: what is 1 + 1? They have to put the number 2 in the answer which means unless they're actually reading the form instead of using automated software, it should help prevent most spam.