Incorrect syntax near ','.

string query = "INSERT INTO Customer (custID, title, firstName, lastName, address1, address2, address3, address4, postCode, phoneNumber, mobileNumber, email, fax) VALUES (@accountNumber, @bxTitle, @bxFirstName, @bxLastName, @bxAddressLine1, @bxAddressLine2, @bxCity, @bxCounty, @bxPostCode, @bxLandlineNumber, @bxMobileNumber, @bxEmailAddress, @bxFaxNumber)";
SqlCommand cmd = new SqlCommand(query, sqlCon); // sqlCon is of type SqlConnection

// Here for a string column that do not accept a null value:
if (!string.IsNullOrEmpty(txtBxAccountNumber.Text)) {
   cmd.Parameters.AddWithValue("@accountNumber", txtBxAccountNumber.Text);
}
else {
   throw new ArgumentException("AccountNumber field cannot be empty");
}

// OR (if custId column is of integer type)
int accountNumber;
if (int.TryParse(txtBxAccountNumber.Text, out accountNumber)) {
   cmd.Parameters.AddWithValue("@accountNumber", accountNumber);
}
else {
   throw new ArgumentException("AccountNumber is not a valid integer value");
}

// ... and so on with all other parameters