Mine

<script language="javascript">
function validate(contactform) {
if (contactform.elements['name'].value=="") { 
alert("You must enter a name so we know who you are!");
return false;
} 

if ((contactform.elements['email_address'].value=="") && (contactform.elements['phone_number'].value=="")) {
alert("You must enter an email address or phone number so we can contact you if we need to."));
return false;
}

else
return true;
} 
</script>