function setCallbackName() {
    var field = document.getElementById('callback_name');
    if (field.value == "") {
        field.value = "Your name";
    }
}
function setCallbackPhone() {
    var field = document.getElementById('callback_phone');
    if (field.value == "") {
        field.value = "Your phone number or email";
    }
}
function clearCallbackName() {
    var field = document.getElementById('callback_name');
    if (field.value == "Your name") {
        field.value = "";
    }
}
function clearCallbackPhone() {
    var field = document.getElementById('callback_phone');
    if (field.value == "Your phone number or email") {
        field.value = "";
    }
}
function validateCallback() {
    var callbackName = document.getElementById('callback_name');
    var callbackPhone = document.getElementById('callback_phone');

    if (callbackName.value == "Your name" || callbackName.value == "") {
        alert("Please provide your full name.");
        callbackName.focus();
        return false;
    }
    if (callbackPhone .value == "Your phone number or email" || callbackPhone .value == "") {
        alert("Please provide your phone number or email.");
        callbackPhone.focus();
        return false;
    }
    return true;
}