After carefully reading the script pin_number.lua I could see that I could use pin_number = voicemail only that I did not want it to ask me for the extension number but to take it from the variables so I modified the line
user_ext = session
layAndGetDigits(min_digits, max_digits, max_tries, digit_timeout, "#", "phrase
in_number_enter_extension:#", "", "\\d+");
with
user_ext = session:getVariable("caller_id_number");