Edit in JSFiddle

$("#btnSubmit").click(function () {
    //validate mobile
    var phone = $("#txtContactNumber").val();
    if (!validatePhone(phone)) {
        alert('Invalid Contact Number');
        $("#txtContactNumber").focus();
        return;
    }
    
    alert('Form submitted');
});

function validatePhone(phoneText) {
    var filter = /^[0-9-+]+$/;
    if (filter.test(phoneText)) {
        return true;
    }
    else {
        return false;
    }
}

<input id="txtContactNumber" type="text" />
<br /><br />
<input id="btnSubmit" type="submit" value="submit" />