Javascript phone number validation

I need to validate a phone number in javascript.The requirements are:

they should be 10 digits, no comma, no dashes, just the numbers, and not the 1+ in front

this is what I wrote so far

function validatePhone(field,alerttxt) {    with (field) {        if(value.length > 10) {            alert(alerttext);            return false;        }        for(i = 0; i < value.length; i++) {            if(parseInt(value[i]) == NaN) {                alert(alerttxt);                return false;            }        }        return true;    }}function validateForm(thisform) {        if (validatePhone(phone,"Invalid phone number")==false) {            phone.focus();            return false;        }    }}<form action="post.php" method="post" id="contactform" onsubmit="return validateForm(this)"><ol><label for="phone">Your phone <span class="red"></span></label><input id="phone" name="phone" class="text" /></li></ol></form>

but, obviously it doesn't work.How can I write the validatePhone() function so that it works?